well, yeah, you can...
but 7 strings feel fucking weird. Cool, though. No reason not to get one, but I could never see having one as my main guitar...Megadeth's new guitarist (that guy from nevermore or something...Chris Broderick, i think) uses one, and he shreds like a beast.
As far as tuning to A, that's a little nuts. Sure, it'll be heavy, but you won't be able to play anything standard tuned. Unless you absolutely need the most abysmally heavy guitar sound, a set of 10s, heavy pickups, a nice amp, and tuning to D will cover you.