I always figured the normal playable bass range was from the low E up to the 12th fret G, so that’s E1 through G3, then due to dropped tunings, I added D1 and Eb1, and later down to C1, although the C1 and C#1 don’t get much use.
As to note length, you want it to cover the longest note you’re likely to encounter 90+ % of the time. I found 2 measure at 120 BPM was a good compromise, but I have made them shorter in some kits. I think you’d want to be able to cover the single note 60 BPM songs, thus the 2 measures at 120 BPM.
Choke groups are not a bad idea. If you are planning on never having two bass notes play simultaneously, you can put all the bass notes in a choke group and set the polyphony to 1. I find when composing I sometimes want bass power chords, so polyphony 2 would be acceptable.
I stop notes short by having the basses as non-percussion and then BB responds to the note off. But, if you want to leave them at percussion, and you use a choke group, inserting a velocity 1 note, after the initial note can work. You could record a nearly silent short wav file, and add it to the kit as your C1 (midi 0, or whatever is going to be outside of your normal kit range.) Be sure it is added to the choke group, if you go with polyphony 1. Then you can trigger that when you want a cut off note.
Good luck with your endeavor.