My strings on my G-400 feel like crap so its time to change them.
I usually play anywhere from Standard to Drop C.

What gauge can give me a playable, tight feeling for standard, but won't be too loose for drop C?

Would 11's work or are they too heavy for standard?
I use 10's on mine and they feel good and tight in standard. I'd imagine 11's would bite a bit on the bends
I think maybe a set of Ernie Ball Skinny Top, Heavy Bottoms would work for you.
The plain strings are light enough to bend easily, and the bass strings are thick enough for drop C.