I built my F90 right out of the book, using an OS .91 FX and pitts muffler. Plenty of room to move the battery pack around to change the CG. I had the CG about 1/4" further back than recommended and it flew fine. I think the Saito is pretty light. You should be able to mount the engine tight to the firewall and adjust the engine cowl accordingly. I used a GP engine mount on mine, I didn't like the H9 one that came with it.

The elevators are independent on their own servos, you don't connect them together, you program mix them with your computer radio. I used a 9C, with standard flaperon setup on channels 1 and 6 and the elevators on 2 and 7 using program mix to match the elevator servos for centering, ATV/EPA and everything worked fine. I have seen several on Saito 100's with all the servos in the tail and they flew fine. I did mount my 6 Volt battery pack up on the side of the engine box initially, so the plane balanced as required, with no weight anywhere. I used Futaba 3010's on the elevator and 3305's on the rudder and ailerons. The 3010s are about 90 oz. in. at 6 volts, and very light and worked fine on the elevator. The 3305 has metal gears and is a little heavier and is 125 oz.in. at 6 Volts.
We have guys flying these things at our field with OS pumped Hype .91 helo engines on 30% nitro and JR8411 servos and they rock. I was happy with mine with the standard OS .91 FX and less expensive analog servos.

You may want to get it all together except the rudder servo and check the CG. You can rest the rudder servo above the mounting bay on the horizontal stab, to see how the CG is before you do final mounting. The rudder servo on the tail is so much easier than pull-pull, believe me.

Good luck with your plane, it rocks, flies like a small IMAC bird and 3D's pretty darn good too.