I don't like your fire teams idea, guard just aren't tough enough to do be effective at that unit size.
I had it more in mind for cityfights than normal open-board battles, where cover is everywhere.
I doubt 5 lasguns on their own could ever be of much use though.
1) IG can do this two ways. In the macharian crusade rules, you can have 'advanced tactics' where grenadiers and storm troopers can switch hellguns for bolters +10 points a unit, and can use combat squads.
2) Cities of Death (It might have been the old Cityfight, not sure) IG automatically get this ability.
And you are missing the big picture. It's not 5 lasguns per unit. It's one squad with a special (melta or plasma, and in cityfight, flamers would be useful too) and the other with a heavy weapon, to cover the advancing unit. Very effective.
And Rmyr, what do you mean fleet and fire? Guard can't fleet, and if you run, you can't shoot... right?