I think two girl cats have a better chance of not getting along...my two didn't and others have told me this as well...I have one of each now and they get along swell! BUT it all depends on the cats I guess.
The SPCA and most rescues should be able to tell you which cats get along with dogs and other cats...they usually try to find out to help prospective adopters.
Slow intros (new kitty in a seperate room to start) and make sure you dote of your current pets too so they don't get jealous
As for the declawing issue...usually you keep claws pretty short by clipping them, so kinda the same thing except without the pain of declawing, so I don't really see it as an issue...especially if the cat you are looking at already gets along with other animals