An older, lower capacity battery will not suck life out of a newer one in a parallel system.
Old and new batteries all rest at 12.6 volts, so no current is going to move between them.
A used battery may have only 1/2 the capacity of a new one, but it along with a new one, will add to the total capacity.
On the other hand a bad battery could drag down the good ones. So if using a mix, just keep an eye on the older ones condition.
Keep in mind that anytime your BIRD kicks in you are combining a house bank with a chassis bank.
Most of us don't change all the batteries ( chassis, house ) at the same time. We certainly don't use the same type.
You should not change a 6 volt battery in a series set. Change both and save the good one for the next bad one.