It has been a while since you asked this question so you might have already figured out an answer or given up. If you are still interested I'll give you what I know about this area. When using a sequencer like Cakewalk, for instance, it maters not what track you assign to what instrument. For the most part it seem that everyone uses track 1 as a control track, and track 10 is used for drum tracks. For a long time track 16 was used for the drum tracks, but now it's track ten. For the sake of less confusion, I always try to make the sequencer track match the midi track. (Seq trak 1 = midi track 1, and so on till seq track 16 = midi track 16. Then if you have a second midi string (B), seq track 17 =(B) midi track 1. etc.)
Assigning the programs to the tracks is simply your choice. You could put the bass part on seq track one if you want, and then put strings, for the melody, on seq track 2.
The bass instrument would be set to receive on midi track one and the string instrument would be set to receive on midi track 2.
Assignments of the voices, (or patches, as they are called) is done with PC commands in the event window for each seq track.
I hope this is the type of information you wanted to receive. I won't go on till I know we are going in the right direction.
Cheers
Majik