I don't know much about the breeding Rummynose tetra since I never owned them so I never did research about their breeding condition seriously. But if my memory is correct, they do scatter eggs in big group. Like 10 or dozen of them. Anyway, you can go to one of the search engine and type in "breeding Rummynose tetra" and you should get tons of result. And just read them some. I know some would be useless but there should be some reliable note or journal to follow or get some basic idea from.
And you mentioned about the oyster shell cave. You might need to keep eyes on the water condition since any sea-shell to be known to add the hardness to the fresh water tank.