Really, really, really bad ideas on both counts.
A standard shotgun shell would definitely blow up a plastic flare gun- that's why flare shells are a good bit shorter than standard shotgun shells, the flare guns have shorter chambers too.
Flares have a bad habit of sticking in standard shotgun barrels- notice how much longer a shotgun barrel is than a flare gun barrel? And flare guns don't have chokes either. I've been told it's an interesting experience to have a flare burn itself out while stuck in your shotgun barrel, it makes a pretty red glow as it heats the barrel. 'Course, it also ruins the barrel.
lpl/nc