i too am wary of dividers as mine kept jumping too. i ended up making one of my own out of an old UGF

i also dislike dividers as it cant be good for the bettas to be constantly flaring at one another. I would personally wait until you can get another tank. Although bettas do origionally come from thailand, were i believe they lived in muddy rice paddys, i dont see why we should still confine them to such small spaces. All of my bettas now have 10g+ tanks of their own, a few of them have some cories with them, and they seem to be much happier
As other have mentioned, male bettas are distinguishable by their tails and also *usually* colour. Male bettas will have a much longer tail, and with breeds such as crowntails they will have a kinda 'forked' look about them. Veiltails dont have the 'forks' or 'spikes' at the end of their tails, but they will be much longer than those of a female betta
i would suggest you wait till you can get another 10g and keep him in there. If you just have to have something in there with him, cories are a very good idea