Doing more than 1 workout regime at a time?

Sure you can. Challenging your muscles and changing up your routine actually help. I do strength training with machines/body weight/TRX, classes like Jillian Michaels Shred, cardio and vinyasa yoga.

It would not hurt you most likely. You need to learn your body and see. If you are getting tired more easy, losing strength or endurance when working out, having sleep problems, etc then you might be over-training and need to slow down. But if you keep building muscles, getting stronger, being able to do exercises easier, more reps and progressing overall, and feeling great then you are fine. So there is not maximum amount of exercises you can do, you can do as much as you want, if you can recover from it.

Now the thing is that doing "too much" might be unnecessary work what would not make much difference. Depending on program you are and your goals of course. Doing more exercises will burn more calories so could help you lose fat to get those abs cos it may put you in bigger calorie deficit. But there is only as much muscle strength and growth as you can stimulate. And doing too much would just burn extra calories and train endurance. It really depends on what kind of workout you are. Most of the workouts what are designed for specific purpose will be designed to get you the best results you can get so training more would not make much of a difference, cos you already getting the best results anyways. In fact, sometimes doing more can even slow down progress, more is not better better. Rest timing is important part of all it.

I would suggest to stick to one type of workout for one thing. For example if you are doing stretching workout then let it be your only one but you can add something else, like cardio. If you are doing endurance workout then let it be your only endurance workout but you can add strength for example. But don't add another strength workout if you are already doing one strength workout for example. That being said, you can, if you are just doing it for fun. But be aware that instead of those 100% you might have gotten if you would not have added it in you might only get like 80 or 90%. It does not always slow down your progress when you add more, but sometimes it might, even if you re not over-training. And of course depending from workout sometimes it might speed your progress, if that workout is not ALREADY designed to squeeze out 100% of you.

Will it hurt my muscles too much?

I'm hoping to start doing yoga (not sure what the differences are), a 20 minute leg workout, an ab routine (20 minutes), weight lifting, and Jillian Michael's video. I have no idea when I'm going to have the time for like 4 hours of working out though, but I'll figure it out. Because I have target areas:

I want my leg/thighs to have bigger muscles, my abs to show on my stomach, and my arms to be stronger (currently unable to do 10 girl push ups). I want this to be accomplished in 6-8 weeks.

If I were you, I would do something like:

Monday - 30m yoga, 20m legs,
Tuesday - weights, 20m abs
Wednesday - Jillian Michaels, 30m yoga
Thursday - 30m yoga (low intensity "rest day")
Friday - Jillian Michaels, 20m legs
Saturday - Jillian Michaels, 20m abs
Sunday - 30m yoga (low intensity), optional weights

assuming that the JM video is mostly cardio, you've got 3 hours solid cardio, lots of stretching from yoga, a couple of sessions per week on legs, abs and weights (assuming you focus on arms), then i might do something like 3 sets of as many push-ups as you can do in the morning every weekday just to focus on your push-up goal

I do Centurion and Parkour. Do as many as you want. Mixing it up is great.