As long as you keep the wood fully submerged it should work. I've kept betta sororities before and since they've been raised together as long as you make certain there's enough hardscape and plants to break up lines of sight you should be all set. They will get more aggressive as they get older, towards ALL tank inhabitants, it's just their thing. I would suggest 3-5 as opposed to 7, though, considering tank dimensions on 29 gallons so they can set up their own "safe haven" spots to feed in should they get aggressive as they grow. As for other tank inhabitants- neon green rasbora, any of the small rasboras really, small bodied danio and tetra also work well, as long as they are not long finned or nippy. Cories do pretty well in betta sororities. The main thing with keeping multiple betta together is giving lots of space for them to establish themselves, giving them plenty of hardy, quick moving dither fish to pick on and using hardscaping and plants to break up sight lines and give everyone places to hide. Good luck!!
Oh- just remembered this from one from my betta breeding days- potted plants!! They can be moved where you need them when you notice sight lines. I'd also invest in a couple breeder boxes (not the clear ones) that attach to the tank at the water line to separate them if you have issues- it's much easier than setting up separate tanks and since you are going through a local breeder you should be able to return trouble makers pretty easily and quickly so you don't really need a permanent fix. Really hope this helps
