Rimas,
I have most of the gear you are talking about
: AP900 mount, Tri36M tripod, FSQ-106ED and a few different cameras including a Starlight Xpress H-18 (KAF8300 mono sensor). It's all great stuff and I'm very happy with my setup. I don't have a permanent observatory, so it's all portable use.
Camera: the KAF8300 sensor is a good match to the FSQ. KAI11000 and KAF16803 are really great too but more expensive. I can't comment on the ST-i. I use a Starlight Xpress Lodestar which works well but requires calibration frames in Maxim for best results.
Accessories: I'm using an Atlas focuser with Astro-Physics 2.7" thread on the camera side so I can't offer advice on adapters for your setup. The Atlas is a great upgrade for accurate focusing. Some people have had problems with tilt on the FSQ focuser with heavy cameras.
To mount the scope I use Socal Astro rings attached to dovetail plates top and bottom. Unfortunately, I don't think these are currently available:
http://www.socalastro.com/images/rings/index.htm
A Tak tube holder with one of the BT Technologies dovetail plates mated to an Astro-Physics DOVELM162 saddle would work too.
And, yes, you can use the AP900 standalone without a computer.
Cheers,
Rick.