If your chins are currently housed together, I see no problem in transporting them together...
Regarding the rabbit carrier--because rabbits carry pastuerella (sp?) which is deadly to chins, I would make sure that if you're going to use the rabbit carrier, that you clean and disinfect it well before using it.
I think the cottage-cage would depend on how often you're going to be going there. I know that I personally would not care to move a large cage back and forth and back and forth. JMO, but if I thought I was going to be spending enough time at the cottage that the chins needed a cage there, I would buy them a separate cage and just leave it at the cottage, so all you would have to do would be to get the chins in a carrier and take them, and not worry about bringing a cage every time as well...