what engine are you starting?
I have a perkins 4.154 62hp engine. I use a standard group 31 starting battery with 1000 CCA.
I would combine all the 6v batteries into one large bank and add a starting battery.
all chargers that I know of want to only charge one kind of battery, so don't mix in AGM with your wet cells
Buy calder's boat owners mechanical and electrical manual. Detailed explainations on what, how and why for all this stuff. Or one of Don Casey's electrical books