Look under Optional Themes for some themes.

The big constraint is working with the assets I give you!

Well, if you play CHILL MODE, the rules are very relaxed, do whatcha want.

If you choose the other modes, you should not change the data (change pitch values), but if the engine you use supports playing things faster or pitch-shifting itself, that is probably okay. But the spirit is don't change the actual data, nor add detail to things.

If you use Pico-8, very much so. Coding is easier in Lua, and you are given all the assets you can use, already in Pico-8 format. And the community is very helpful and kind to new folks. 

FOR HARD MODE:

You can change the color of already existing colors with PAL. 

You cannot add detail.

The only way to change size is with SSPR drawing the sprite bigger. You should not be editing any pixels.  But scaling the existing sprite to look bigger onscreen with SSPR is fine. You can also  use parts of sprites with SSPR or use CLIP.