I love the metal tub idea! I think I'm going to go with those for holding bins. Could I put a hole through the side and throw in some watering sippers?

Very great idea. How many can you wind up keeping in each bin at a time and do you have a problem adding a new litter of males to a tub of older males (same goes for the female tub) as long as they're not very tiny?

Or do fights break out in existing groups only if you have both sexes?