I always see this "Keep them the same" Myth
Now if you are putting pairs of six volt batteries in series then each of the pair must be the same, same age, size, whatever.. However that is because they are two halves of the same 12 volt battery.
But when you put them in parallel.. Unless you are putting like a 10 year old battery up alongside a brand new one there is not much danger (and that 10 year old is likely toast to begin with)
"If one is lower voltage it will take down the other" Right.. Now tell me, how is one goign to be a lower voltage.? I mean they are PARALLEL. so while it is true that when you first hook them up one will indeed be lower than the other... That will last for one discharge cycle only.
UNLESS, of course, one battery has a shorted cell, in which case it's time to replace it.
The only thing is they should all be the same type
If the one you have is flooded wet where you can (And shoudl) add distilled water.. The other one should be too
If one is AGM. the other shoudl be too
Maintenance free (But not AGM) again stick with the same type
But as for size, age, capacity.. The batteries will work that out. I have no problems putting a Group 31 and a Group 27 beside each other. Work well..
Just so long as you don't have a shorted cell (Which is one type of bad battery)