So I have a Battle Sister Character, Rank 4, Both Celestian and Seraphim both say that they require rank 5. So when I reach Rank 5 I replace my current rank with with one of them.

Once I reach rank 6: Can I take the other? is there a limit? (Provided I have all the required attributes and talents)

Usually any more than one is frowned upon and starts smelling of trying to game the system.

If your GM is okay with it though, go for it. Just remember, you loose access to all the advances on each normal rank that you replace with the alt. You can only purchase the missed advances with GM permission as elite advances.

Since most alternate ranks say "can be taken at rank x or higher", you can exchange any rank you want. The downside would obviously be that more and more of the advancements in your main career become more expensive. As for multiple rank exchanges, I really don't agree with my predecessor - if there's a rank that fits with what you have in mind for your character, take it.