There are a LOT of questions there, that really can only be answered by you... but there are some guidelines that you could try to follow.
First, try to find fish that are compatible with your tap water conditions.
Second, focus on a single fish species that you absolutely need to have, and choose tank mates that are compatible with that species.
Third, research, RESEARCH,
RESEARCH.
Once you have settled on the fish, then you can start to look into the other stuff, like plants. That's a whole other ball of wax (the planted section would be worth a visit). In the meantime, I would say that you should have a good long read through the
Beginner's Resource Center thread.
Have you cycled either tank yet? If you haven't, then focus on the fish-in cycle for the betta tank, and focus on the fishLESS cycle for the 55 gallon.
Post your dimensions to your tank (LxWxH). Most fish have specific needs regarding swimming space, besides the obvious space to subsist. Just because the fish "fits" in the space, doesn't mean that it would fare well in some tank. The amount of activity by that fish, it's need for a shoal or its need for independence are huge factors to investigate. The possibilities really are endless. That's why I suggest you focus on ONE fish species. If that is the oscar, that will pretty much preclude you from having a community. The world of cichlids is a very colorful one, but oscars get so big that with that size tank, you won't have much room for many others. In fact, most oscars require more than 55 gallons, from what I've read. Good luck and feel free to post questions here along the way. The members here are very generous in their advice!
