Well there are no real 'true deep cycle' flooded group 27 batteries. With Rolls-surrete being a possible exception
AGM's Like Lifeline, perhaps some others, do qualify.
If one wants to make use of older flooded 'deep cycle' with new series GC-2's in parallel, it is likely the 27's, if still healthy, will have lesser resistance and higher resting voltage, and will essentially support more of the load and take more of the charging
amperage, and be used up faster.
But the GC-2s in such a situation might never get fully charged, or be consistently overcharged, and when the 27's die and you want to replace them with new GC-2s.... New and old GC-2s will be somewhat similarly mismatched, but a bit less so, and Unideal will continue, perhaps with little consequence, or perhaps not.
Also in my experience, marine
flooded 27's, take forever at absorption voltage for specific gravity to max out, in deep cycle duty.....Likely far more time than healthy New flooded GC-2's when held at the same voltage. All the flooded 27's and 31s I've treated like true deep cycle batteries, when I checked specific gravity, proved they were unhappy in such duty, and even if the solar got them to 14.7 by 10Am and held it until late afternoon, the specific gravity never maxed out at predetermined maximums. Even when I set float to 15.3v, after 4 hours at 14.7v, they remained below prerecorded maximums.
The flooded 31 I treatred best achieved only 550 deep cycles before it was just not worth the efort to keep cycling it deeply, but in shallow cycle duty it lasted another ~300 cycles with minimal care regarding charging
properly. Used a LOT of water
, chewed up the concrete under it. Stunk like a fried rotten egg for hours each day.
Exactly how new and old and different type interact in use has many variables, variables best eliminated buy not mixing and matching in the first place.
But its hard to dismiss batteries with life left in them just because they do not match with what would be ideal. I've got 4 such batteries on workshop floor, feeding on solar, taking some load off the grid, and the 15 amp 120vAC breaker.
Mainesail's 'what is a deep cycle battery' is good reading, when someone believes their flooded 27, which says deep cycle on it, is an actual deep cycle battery.
One can deep cycle a stating battery, it does not make it a deep cycle battery, only a battery being deeply cycled.
When I had one flooded bank and one AGm
bank, I had two 1/2/both off switches, one for loads and one for charging sources, and could cycle one and charge the other, easily enough, then swap them the next day. Lots of room for human error though.