I honestly think boarding is best kept as a marginal part of the game and should use existing mechanics,
An idea I had - "Storm Trooper Boarding Party" - Offensive retrofit - medium or large ship only - black crit (not sure - ISD2 should be ale to take it! - if the defending hull zone has no shields, turn the damage deck cards over one at a time, until you have two crew results. Apply both to the defending ship.
Practically speaking, boarding a VSD or ISD shouldn't be a realistic proposition for the rebels. I carries an entire legion of stormtroopers! Also I suspect Imperial ship internal spaces are very heavily fortified, as much to suppress any internal disloyalty as anything else. And Imp vs Imp boarding would be better classesd as "siege warfare" rather than boarding.
So boarding at most should be limited to hit and run sabotage attacks that can basically be resolved as ordnance.