These are my answers to the questions you posted a while ago...
1. The GP-DX uses a dovetail to connect the mount to the telescope.
The dovetail base on the GP-DX requires a dovetail plate to connect
the rings that Vic supplies. This dovetail plate is sold by Orion and
I think Vic sells them too OR you can make one yourself with a piece
of aluminum of the following size 8" x 1 3/4" x 1/2" thick. If you
want I can post a photo of mine (homemade).
2. The "subjective" question is not that subjective as you think.
What is subjective is the term "astrophotography". If you plan to do
planetary and lunar photos you will want the longer focal length but
if you want to do deep space and stars you will need more apperture
and shorter focal length. In any case the better color correction of
the design will provide the better results. If I were to order a
Stellarvue scope right now (and I soon will) I would order the 102EDT
(f6.1, 621mm FL) or the 102APO (f8.0, 816mm FL). The last one is a
serious scope for "serious" astrophotos but Vic has advised that the
wait period will be long so I would go with the 102EDT.
I live in a city that has a BIG skyglow (light contamination of the
sky). This has not created an invincible problem for me but it does
create a serious handicap. For long focal length I have a Celestron G-
8 SCT. For medium length and portability and great handling I have
the Stellarvue 80mm f/9 (I am using it for Daytime photos too!) for
deep space I use the G-8 with the Celestron f/6.4 focal reducer BUT
it never compares to what I would get with the Stellarvue f/6.1-ED
triplet. The Crayford Style focuser alone is a PLUS PLUS.
3. I use separate power sources. For the GP-DX I use the battery pack
that comes with the motors. For the camera I use the internal
rechargeable battery etc.. Why? believe it or not there are less
problems. (I hate those cable tangles).
Hope this helps.