To be honest, I have never kept these myself so wouldn't be all that qualified to answer those questions.
I know that all the n. sexfasciatus will crossbreed as they are the same species. However, I have only seen records of the gold sexfasciatus actualy being bred in captivity. I'm not sure whether this is just sheer coincidence or more significant. The n. tetrochephalus is probably also quite closely related to them but I doubt they'd hybridize. Still, the simialr appearance may be a good enough reason to not mix them.
I can't help with the cost of them sorry. They are also quite hard to come by so you'll probably need to ask your LFS to order them for you or purchase some online or something.
When it comes to keeping them with other fish, that'll work with equaly aggressive tanganyikan cichlids but stick to only one pair per tank. You'll probably have to get quite a few and let a pair form (and remove the rest) as they aren't realy possible to sex accurately until they are fully grown. Also, if they breed, they'll become extremely territorial (like most other cichlids) so be prepaired for this and make sure there are enough hiding places for their tankmates to retreat to.
Hopefuly someone will come along who's had more experience with these 2 species than I.