Jack, personally I'd stick with the 2 house and one start. If you do take one out, take out one of the house so you will still have a "start".
As far as a smaller battery, I have been using a "U-2" lawn tractor battery in my Calibogie for a couple of seasons with no issues. I am starting a 60HP Yamaha and have a couple radios going too. Plenty to crank an A-4. A friend of mine is using one to start a SBC 383 with 11.5:1 compression in a hot rod and it starts just fine.
Dave Neptune