Such a simple little change to the rules, swapping the players shooting phases, could lead to some quite intense firefights. I would try this but the guy I would play against would no doubt take a deathwing army and I don't see any chance of winning.
Then again, I'm a Guard player. Not winning is just as fun if you throw enough bodies at them. Maybe this would be a good time to try out the foot horde.
It sounds pretty interesting and since Marines are supposed to be storm troops and guard fit the bill for desperate defence it could have a good feel to it. To be sporting you could have a beacon that the marine player must deactivate before turn whatever, or he needs to table your guard without going over a casualty limit for his marines.
"True, true. These are rather dark times. How fortunate for me, then, that cats can see in the dark."