i switch babies around back and forth all the time.
Thing to keep in mind.
the biggest younger kits go in with a litter that is older than they are.
the smallest older kits go in with a litter that is younger than they are.
the aim is to keep kits about the same size together so they can evenly compete for food.
You must keep an eye on the condition of the doe and her ability to care for them.
MOST meat type does (so 7 + lbs) can care for up to 10 babies, but not all can do this...but all SHOULD be able to handle seven with no issues.
Most fancy (HL, MR etc) types can handle 5-8
most dwarf types can handle up to 5.