Today, after listening to bloodbath, i decided to tune my Gibson SG special to A standard...it sounds fairly brutal, but unfortunatly, it sounds a lil out of tune despite my tuner's best efforts.. I've got Ernie Ball Power Slinkys on there atm, what strings should i get, and/or can my guitar even handle the tuning with its scale length?
Yeah your guitar can handle that tuning, but you might need to make a couple of alterations to the setup.

First off get some heavier strings, it might be worth looking into a specialist baritone set of strings, something with around a 60 gauge minimum for that low A. Then some adjustments may be needed in terms of action and such, but those are simple things really.

Or the alternate is to buy a 7 string and simply detune a step, both options have their advantages over each other.