6.5's fit just fine. you will always need an adapter, but it's plastic man, just screw into the mount it can be done with pressure and a screwdriver.
however you're right about corrosion. battled it on all my doors. a lot of electrical tape and even pain on rubber insulator has been the only cure.
2009.5 G8GT Premium OnyxRed Interior
corsa catback, vararam intake, cortex tune
custom audio, sharksfin, HIDs, LED Fogs, 20" MRR GT1's