If we decide to do a reverse wedding is it okay to ask the guys to pay for these cheap bridesmaids dresses?

As long as everyone in the wedding party agrees to this wedding switch-a-roo you guys are doing then that's fine. I wouldn't make the guys wear dresses if they didn't want to though. If the guys agree to wearing dresses sure you can have them pay for it. Normally they'd be renting a tux that they'd never wear again so what's the difference if they buy a dress? Between the tux and shoe rental the guys paid I think $120 for their wedding attire in my wedding so your dress is even cheaper.