I've never fostered kits over to another doe but I know I've read where @Pastor Dave has mentioned it maybe he'll have some feedback on the subject... I do breed my does in pairs for that reason but I've yet to have a small litter that deemed it necessary
I've started culling to a litter size of 8 based on advice I relieved on here with some considerable success the kits grow out faster & seem to be healthier... I've done it with 3 litters so far & the results seem to be consistent.