I would definitely do the saltwater in the 29 first to see how everything fairs.
By the way, I'm super jelly of you right now lol Although I was nearly this stoked when I got my 33 gallon free from my university because I was complaining to the right people at the right time! Haha
But congrats to you! I wouldn't put any betta's in there though, just thinking about how long it would take them to get to the surface unless it had a rather large footprint. But I would consider Angel fish, smaller cichlids like apistogrammas or rams (my particular fave.) a catfish or two. My favorites were the Synodontis Eupterus Catfish and spotted Raphael Catfish. You could also do some of the larger Gourami's as well!
To decide on my 33 gallon before I even thought about a sorority I had to write out a list of stocking ideas on a word document and switch things around before I could decide lol It took me about 3 months of pure research on compatibility and requirements before I settled on the sorority!