How are the load and charging wires connected on the four 6s? All on one pair or across both pairs? If the former, that pair is working harder than the other "downstream" pair so it might use more water.
I had a pair of those Interstate 232s banked with a pair of Exide 226s for years no problems. Sometimes I added a pair of Trojan T-1275 12v batts to that so I had six batts in the bank and it all worked great. You can do that if they all have similar charging specc. The different AH sizes don't matter if you load/charge across the whole bunch.
ISTR those Interstates had calcium to them while the T-105s have the antimony so they would use more water? Mex said something about that. Lately, the Interstate 6s are 225AH same as the T-105s and an Interstate guy told me they are made by Trojan. So if you had the newer 225AH Interstates they would not be "mixed" :)