So the DM6 module should be outputting General MIDI on Channel 10, which is received by Garage Band and mapped to the selected synth kit. The MIDI note output for the cymbal might be C4 (middle c on keyboard) or D4 or whatever, but its important to note that it won't change when you change the voice on the Alesis kit.
Garageband should display the actual MIDI note received in the blue bottom transport thingy every time the note is received from the cymbal (on any channel). Jot that down and find out how to change the mapping for the synth kit in Garageband. So for example if the crash is mapped to C4 on the Garageband kit, and the ride is on D4, simply change the voice on C4 to be the ride or whatever. Its perfectly fine to have your kit voices re-mapped to any note or all the notes to the same voice.
Unfortunately, I'm not sure Garageband let's you do that. I might be wrong, but try googling around for how exactly to do that.
If you can't do that in garageband, perhaps you can do that manually in the DM6. Again, you'd want to change the note output from the DM-6 from C4 or whatever the crash is to whatever the ride is. This map of general midi notes, might help.
As per "voices are too low" that might be better as a separate question. By voices, do you mean too low on the midi map, or too low in volume, velocity? If velocity, again, you might be up against a garageband limitation. Not sure if you can you increase the global velocity of all notes in Garageband. Pretty sure you can do that in Logic though. Good luck.