So, lately I've been listening to a lot of Alexisonfire, Underoath and Dance Gavin Dance, and I've been trying to learn their songs on guitar. Right now I have really heavy strings (I was playing a bunch of stuff in drop A), and I find it's a bit tough to play songs by the bands I listed, seeing as most of their songs are in the drop D and drop C# range. What gauge of strings would you recommend for those tunings? Also, a lot of their chords/solos are played on higher strings.
That depends on your preferance, some manufacturers make special sets for drop D tunings, where the lowest string is slightly thicker. For drop D you should be fine with most string gauges IMO
Try playing with your current string gauge, if it rattles, buy the next thicker one.
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9s may be too light if you're used to really heavy drop A tunings. My standard set is the good old 10-46 gauge set. Should be fine for Drop D. One step down doesn't require special strings. You may need to re-intonate the strings if it's a guitar you've been doing drop A with.
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Just get a light top, heavy bottom set that matches whatever you normally use for E.
I'd say 10s for drop D, definitely.

Although you could use a capo and play the songs higher up the fretboard. (I do that for drop D songs on my guitar that is in Drop C)
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