[JRTJH]

I'm not sure you can "pair" two GC2 Duracell or two T-105 Trojan batteries (215 amp/hr) with your existing GC15 batteries.

While I can't say "it won't work" I will say that it's similar to pairing a GP24 battery with a GP27 battery (both 12 volts, different amp/hr). If you do, the two batteries will either:
A) burn up the smaller battery while continuing to charge the larger battery, or
B) never fully charge the larger battery if the charger turns off when the smaller battery is fully charged. That will eventually damage the larger battery, making it incapable of accepting a full charge.

IMHO (and as I said in the first and second sentences, I'm not sure) you can't "pair" 4 batteries in a "series/parallel bank" if they are two different sizes, two different charge capacities and two different amp/hr ratings.

I don't think you'll get maximum benefit from the larger batteries and may overcharge/damage the smaller batteries at the same time.