can anyone tell me if the A600 with Kick 1.3 is 100% compatible with the A500?
No it's not. You don't have a numeric keypad which was used by some flight simulators at least.
Also there are probably some games that don't like ECS.
The lack of c00000 memory might stop you from running a game that needs 1mb.
But there are some games that don't work with 1.3 anyway & need 1.2.
The small case of the a600 is nice though.
If you used WHDLOAD then the games are usually patched to work better on newer hardware.