hi and welcome!
- I have an animation and I want to make it loop in-between two specific frames once it reaches the first of these two frames. How can I do this?
You should customize Animation class see Slick source code for this. Best way is customize your spritesheet and make your loop so.
- Should I use the Marte Engine to create enemies? What about Phys2D, should I use it for collision detection?
You can use Marte Engine for make a platformer, see MarteEngine Fuzzy example on github.
Phys2D is a different stuff from ME, take a look to examples of both and decide. I think Phys2D have more you want to do a platformer