Hi Chris,
You need a expanded (64k) 600XL for the Cartridge interface.
This is needed because the MyIDE-bios is loaded into the upper 16k RAM (of the needed 64K).
A 600XL only has 16K at the lower end of the address-range.
Development in a 600XL bios-support is too early to make it worth buying one without expanding your XL.
Pin 20 always have power, the early production didn't.
Cheers,
Sijmen.
P.S. If you can solder, there is a easy upgrade internally to 64K.
You only need two 64Kx4 (4464 RAM-IC's) and 3 jumper-wires.
http://www.atariarchives.org/cfn/05/07/0026.php