18f trying to gain muscle, ADVICE PLEASE!!!

Erm, the p90x I've seen has you following their video; it's far from what experts would generally suggest is 'optimal' to gain muscle.
I'm not saying you won't get results, just that they'll likely be a lot slower to come.

The big thing I see with using 'real' weights over body weight is that it's very easy to measure continual progression at a specific rep range.
With body weight you can either do the same move with more reps, or adjust your position - for instance moving your feet on to something higher for a push up (though that will also change the exercise a bit each time too). It can be a bit troublesome to get consistent progression and likely run out of way to work legs in a low rep range that I've seen

There is always a harder exercise. One can also augment their body weight with a small amount of weight. Information becomes scarce though as you get stronger as so few people are capable of doing the hardest exercises, of the few that can only a few of them ever writes about or notes the existence of the hardest exercises.

Measuring continual progression in a specific rep range does not lead to muscle gain progress. Muscle gain progress leads to measurable progression in a specific rep range.

Progress comes from your diet.

Because muscle is only gained in a reasonably measurable quantity in a calorie surplus, any statement about results can only mean that you will gain less muscle and more fat when bulking when doing certain types of exercises. The reason it is never framed that way is because in practice that is incorrect, people doing BW work or P90X work do not bulk overly inefficiently, in fact the opposite is often observed.

I;m taking a really great protein powder. It's by Twinlabs called cleanseries.

I love it because it has all the necessary amino acids from whey isolate, is sweetened with stevia and gluten/soy free. A lot of protein powders are crap, either they have sugar or other crap you don't need. I'm vegetarian so it is harder for me to gain and retain muscle. Vegetable/nut protein is not assimilated the same by the body as animal protein. So now, I'm getting my daily protein and gaining muscle is easy. I noticed a difference in just a handful of days. And as I have experienced in the past with whey, there is no noticeable bloating/gas etc. I drink one before working out with 1c skim milk (bananas, cashew butter thicken it up too ) But, it has definitely helped. I do the machines at the gym to build muscle and then I tone with 2lb and 3lb weights. Anywho, I hope this was helpful to you. Good luck!

Add whole milk to your diet. Get lactaid if you are lactose intolerant. Don't change anything else. If you are going to do P-90X you will need to find ways to increase resistance on some of the exercises such as pushups. You can do this by using progressive resistance bands. For pullups if you can already do at least 10 reps then you can get a belt and tie weights to it to increase resistance.

It is very good that you want to gain muscle weight. Regarding the muscle usage, your muscle needs to be exercised. Exercise fill our muscles with protein, and are the main building blocks of muscle. Now, about the weight gain. You can gain fat that is used in your body, but does not clogs. What I am talking about is called saturated fat. Saturated fats are also called good fats. If your body can get fat to burn rather than the protein, your muscles are sure to build. Coconut oil, The DV for saturated fat is 20 grams per day. There are many foods for saturated fats, Coconut, butter, dark chocolate, cheese, nuts and seeds, processed meat, rendered animal fats, etc. Choose your diet carefully.

Diet is PART of progress in muscle gain.
But have the perfect diet for muscle without the exercise (and the correct exercise at that) and you're just putting fat on in a calorific excess.
Sure with steady state cardio you may still get some small muscle gains, but it'll likely be mostly fat.
As far as muscle gain goes, it's generally accepted that a rep range around 5 is ideal for Myofibrillar Hypertrophy while around 10 reps is ideal for Sarcoplasmic hypertrophy. Of course the human body is very complex and gains be got from all sorts of methods.

I'd say it's much easier to continually to do movements that push a move to a specific rep range with weights as opposed to body weight.

I'd be genuinely interested to hear about a body weight exercise that replicates a heavy squat - appreciated

Apart from people sponsored by P90x, have you got anything to back up that it's very effective for bulking?

The hardest BW leg exercise I know of is a deficit advanced shrimp squat: stand on something small but stable, about 3"-4" tall, with space on the side/behind, grasp your nonworking leg's foot behind your with BOTH hands, squat all the way down to knee contact with the ground.

I can do real good form pistol squats holding more than 1/3 my BW in a front rack, but cannot do even a single rep of a deficit advanced shrimp (though I can do them on flat ground).

A few pistol squat variations that are much, much harder than the basic pistol squat:
1) Hold your nonworking knee tucked to your chest instead of straight out in front of you.
2) Interlace your fingers together behind your lower back
3) Hold your leg out to the side instead of out in front of you
1&2 increase the distance your center of mass is from your knee, greatly increasing the load in your muscles. 3 changes the stability plane putting way more load on the glute medius, significantly reducing the glute maximus contribution (hence increasing the quad load).

Even doing something as simple as prisoner pistol squats (hands behind the head) increases the load a good bit. The more upright you squat, the longer the lever arm to the knee and the higher the load in the muscles (same reason front squats are harder than back squats).

Natural hamstring curls (also called ghetto GHR's) are another extremely high load BW exercise, especially if doing hardcore form; straight as possible in the hips, hands over the head, with below parallel space.

The flipside of natural hamstring curls, natural leg extensions, are also possible, but I've only ever seen one example of it being performed on video with anything beyond utterly pathetic form.

The myo/sarco is a bunch of nonsense pretty much totally unproven. You can't have one without the other, they both go hand in hand. The difference in "look" has more to do with dehydration, carb loading, and body fat levels than any actual physical differences. Muscle CSA (cross section area) has been proven time and time again to be predictive of strength potential, the 'type" of hypertrohy has nothing to do with it.

Perhaps you miss that in P90X Tony specifically says to gain muscle, increase the weight and decrease the reps.

Anyone that says that short rest periods isn't good for growth has no idea of what they are talking about. Rest pause work, with very short rest periods, is ridiculously effective for mass gains.

Staying in a single rep range is about the absolute worst way to gain muscle mass. Barely a step up from doing only straight set high rep work (which, when done pyramid style instead of straight sets, to eliminate failure/fatigue from the cardio system and lactic acid build up, and repeatedly force failure, is also a ridiculously effective mass building technique). The body thrives on a variety of rep ranges, from ultra heavy, negatives/singles/doubes/triples on up to the lactic zone (20+), and everything in-between.

Perhapsyou should pay attention to places where people have logs going that do p90X. There seems to be little issue building muscle mass while doing the program if that is the goal. I'm certainly no p90X fanboy, but a program doesn't get as big as it has if it doesn't work.

Me, I do bodyweight work and loaded calisthenics, have bulked a few times (and am right now) and gain unusually clean.

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It will allow you to add progressive resistance to your pushups.