Intonation Question

A440 is not a measure of key or note, but is a pitch reference. A440 means that a certain A note resonates at 440Hz ( and the lower ones resonates at 220, 110, 55, etc. ) So yes, unless you plan on playing with a group of musicians who want to do something else, like A430 or A444
 
You should definitely adjust the intonation on the guitar saddles. The way to do this is to get a Strobe tuner, like a Peterson, and then play the 12th fret note, and then the 12th fret harmonic. They should be the exact same pitch. When you change the tension, gauge, or action of the strings, you should re-intonate, just to make sure you're in tune as much as possible.
 
I play both 6 and 7 string guitars , the sevens being in standard and the sixes being all different. sevens are a good guitar to get for low tunings because the actual scale length of the guitar is set for the additional string. This means less chance of fret buzz at low tunnings if any. it works the same as a six from e to e but with a low b below. The downside for this is that in order to get c you would have to tune up the b wich is not recomended. My band plays in b so it is no problem.

however, buying a seven is a bit far fetched and you would have to learn to adjust to the extra string. that being said, i have a neck through mocking bird with 13 guage strings on it. Luckily enough i have had no prtoblems with the action. The thicker jazz strings will give you the wound g string(ha ha) wich will compensate for tonality loss on the thinner set. The tunning of course is in b. The downside of thicker strings is they are harder to play, but being tuned down just makes them thicker to play since tension is droped from tuning. Besdides, a litle bit of batle for your ingers would only make you tighter.

another option is the baritone six string. With the scale length set for lower tunnings, they make putting on a thicker set a cinch.

I don't know what you prefer but i use 11 guage strings on my standard tuned guitars.