themes compatibility

Yes, I think so. At least, they probably won't crash too badly if you put them together. Getting them to work together *well* (and looking good) is probably another matter.

I've heard of some experimenting with running a couple of those together, with mixed results.

If you wondering about taking a Litestep theme (for instance) and running it in NextStart; thats not instantly do-able. You'd have to port or convert from one format to the other. They all use bitmaps for their graphics but use different configuration files and support different types of features (different modules etc.)