gaining weight and lifting butt

Hi everyone!

First of all I'm sorry if this is not the correct place to post this.

I'm a 27 year old girl, about 5.3 and 97 pounds.

I've always been very skinny. Until a few years ago I loved my body, legs were fit, butt was tight and almost no cellulite.

The only thing I've always had is a bit of a belly, it's never totally flat, but I don't mind it that much.

With the passing of the years my whole lower area is SO full of cellulite and my butt has lost all definition.

I worked out for some months last year, saw some improvement (my boyfriend noticed too), then stopped for a while and lost it. Now I've started again. I always do the same: go to a class in the gym 3 times a week that is focused on legs, butt and abs. We do 2 types of squats, lunges, hip extensions and a couple more. I do this with some weight, I'd have to make sure how much it is, but approx. 6 pounds probably. I really don't know how many repetitions we make, I guess about 30 for each type of exercise.
Then we do a lot of different abs exercises.

What I'd like:
- To gain some weight, as I think it's healthy for me and look better with more pounds on me.
- Get in shape, mostly lift my butt (I don't mind cellulite as much, as long as the body is in shape, I know you can't avoid cellulite).

My questions:
- Is this workout ok for my goals? Should I do something different, or add something?
- I'd like to alternate with ballet lessons, would that be good for my goals?
- I really like to go running or for walks, but I know that's not very good if you want to gain weight, what do you think?
- Any tips or suggestions would be awesome.

If I left out important information please let me know.
Any help will be awesome, thanks a lot!