Get as many channels on your mixer as you can afford, as you'll need them later when you start buying more gear. It's better to have too many and leave some spare channels, than to always be fiddling around switching cables when all you wanna do is record something or fiddle with a sound.

You might set it up so that cubase sends all your percussion sounds to outputs 1 & 2 on your soundcard - you could then route these through a compressor, and then into channels 1 & 2 on your desk, which'll mean you can pan them hard left and right to get a wide stereo image.

Most modern synths will have both main (master) outputs (l & r) and individual outputs. Within the synth you assign a particular sound to a particular output (eg sub 1, sub 2 etc) and this means you can then process / pan / eq this sound in a different way from the main outputs.

Hope this helps ya.