1. Welcome to the forums.
2. 635 vs 634: 634 requires 2 bays, the 635 1. The 635 is a "gapless" display, the 634 is not. The 635 has keys, the 634 does not. The 635 has LED's, the 634 does not. Both displays are 20 x 4.
My first LCD was a 634 ( I also have a 635). I "agonized" between Matrix Orbital and Crystalfontz, but I finally just had to have the large 634 display. If the 635 had been around, there would have been no "agonizing".
My opinion: get the 635. The actual display is fantastic and you get more bang for the buck. You can also add a SCAB module to 635 and not the 634.
3. CC2 fully supports the 635 keypad, computer keyboard, and LED's.
You can program the keypad (and keyboard) to move between screens, turn them on/off, control Winamp, just to name a few.
Go to the User Screen section, for custom screens, information of how to program the keypad, etc.
Specific questions: search the forums and ask your questions. Someone should be able to answer 'em.
4. CC2 currently does not have the GUI screen editor in place, so search the forums for screens and/or ask what you want in a screen. I have recently seen (pix's) of the editor. Hopefully, you (we) will be able to make use of it later this year.
Hope this helps.