You COULD do that. However you would probably be so frustrated after just a few changes. Those Gerber ones stink!!!!!! They are terrible. If anything I'd just keep using sposies until I got my prefolds in. I personally wouldn't want to deal with the Gerber kind. They are liable to leak since they are basically a layer of really thin birdseye, some polyester batting(polyester doesn't absorb by the way!) and then another layer of birdseye to hold it all in. But theoretically you COULD do that if you so chose.
About covers, Proraps are pretty cheap. Maybe someone has the number for the company because they sell seconds for like $3-4each. Brand new,first quality are lik $8 or so each. That's what I'd go with.
If you sew you could make a few flannel or jersey fitteds with some sewn-in doublers pretty easy. Or turn your prefolds into prefitteds with some elastic. Or make some cheap fleece covers with fleece from wal-mart. That's what I did for our cd'ed little ne when we first started. Now I'm onto knitting soakers and longies for her. It's still slow going but I refuse to buy any wool anymore. I want to knit themall myself!
Meg