im personally not a fan of bettas, i have bad memories of them from when i was a little kid, the cat knocking the little bowl off the table i still say i will never have a betta again
That's so sad, I'm sorry...
You may get away with having a pair of Lamprologus Brevis in that size tank. Beautiful little shelldwellers and something a bit different.
Ah that sucks, you can get them on Aquabid but like you said are a bit pricey. Um, you want just fish or inverts such as snails or shrimp?
I THINK dwarf platies might go in a 5.5 gallon but don't quote me on that. I imagine you could keep at least one. I know my local petsmart stocks them. Maybe endlers livebearer but like I said, I'm not quite sure and endler's aren't as easy to find.
I've actually heard it's not much cheaper than planted makes sense with FOWLR... but... I'll stop convincing you to go over to the dark side, I still need to do that myself.