Definitely feed the leftover veges to the roaches. They will do a great job handling whatever extras you have on hand. I do this and my colony is thriving. They LOVE squash and pumpkins and orange slices too.
If there is adequate room for them to move around you can probably leave the supers in the container they came in. I dumped mine into a shoebox sized plastic bin with a top (holes drilled in the top for ventilation) and filled in with oats and that worked well. I also put a cardboard tube in there which they seemed to like. Then just chunks of carrot or a leaf of the greens you have on hand go on top and changed out every couple of days to prevent molding should do the trick.
You don't need to feed BSFL. I just keep them in the container they arrive in at room temp. They eventually turn stiff and blacken a bit and then after that they hatch into flies. They can be fed off at any of these stages. The flies are pretty slow moving and easy to catch if they get away.