Use the locations called out on the plans.

DON"T USE BALSA FOR YOUR SERVO RAILS! Make them out of 1/4 or 3/8 spuce. Make up 4, 1" square pieces of light ply with a square hole in the middle that fits the servo rail snugly. Slide two light ply pieces over each servo rail before gluing everything to the fuse sides. This "captures" the servo rails and gives more gluing surface so the rails won't pop out from the fuselage sides as easy. I only use a servo tray for large models. You can mount the servos directly to the rails or put them in the tray that comes with the radio.

I don't know what you mean about separating the throttle servo from the others. Line all three of them alongside one another on 1 common set of servo rails.

I usually glue a 1/4" spruce rail in the fuselage to support the fuel tank at the rear. I use Sullivan RST tanks and always drill a large hole in the firewall to put the front of the tank in. Jam foam rubber on top of and along each side of the tank to prevent movement. Run a bead of silicon around the hole after the tank is installed to keep oil out.