Ok, so everybody from your old colony is older? I would recomend that you use virgin (if possible) males from your old colony and get new blood females. Make sure the new blood females you get are as big as possible but definately virgins.

Put them in a freshly cleaned tub / tank. Throw a wheel, some toilet paper rolls, some wood to chew....

Set them up in groups of no more then 1.1 or 1.2. You may have some fighting, but it is not likely under these circumstances that they will fight to the death. Set larger breeding groups up with the babies produced from this pairing.

Let us know how it goes!