I've run 2 layouts: single sidemount port&stbd with a single stern gun in the stbd side amidships turret, and single sidemount with dual sterns. Our battles down south here seem to be closer-range rather than big open ocean affairs, and I went back to the 2 sidemount single stern arrangement after flirting with the other. Stefan (one of the junior members at age 14) runs an I-boat with two single sidemounts and a bow gun in one of the amidships turrets, and he's gotten quite good at sniping with the rifled bow gun.
Side note, all the I-boats I've seen run the plain-Jane 12AH 6V lead acid battery that is so common in the hobby. You can run yours on whatever you like, but the plain old cheapie really does well in I-boats; You'll need one for the morning battle and one for the afternoon, but that should be around $30 all-up, not counting connectors. In the area your moving to, it seems to be a 50/50 mix of Deans and Powerpoles, take your pick. Also, I recommend 1.25" drive props. I used a pair of 400-size motors with those and got pretty much right on speed with 1.25" drag props on the outer shafts. If you use a pair of 550's, you might need to put drag discs on (or use an ESC) but under fast gun rules that'll let you use a 550-size motor on your pump, a good thing

One 3.5 ounce CO2 bottle will be entirely adequate for the I-boat.