I've never kept rams, but I wouldn't stick them with a betta... Depending on the tetra in mind you may go one way or another. Danios like big tanks and chiller water, I would avoid them for now. Properly stocked and managed you could have a decent sized shoal of otocinclus, which is always freaking awesome. Either that or go all south American with a male ram a shoal of neons or cards and a fat shoal of oto or Cory cats
I kept 10 lampeye killies in my 25g. They are really small fish, great for a tank that size. They don't have a very strong schooling behavior (anyway I think 25g is too small for any fish to display real schooling behavior), but they stick mostly together.