Fire is countywide, so for LCFR 6A is dispatch, 6B is low priority calls, but be sure to have the rest of the "6" talkgroups for incidents. Major stuff usually goes to 6D. 6L for things that need their own talkgroup but aren't super significant like calls with fly-outs, gas leaks, wrecks with extrication....
LCSO is pretty well explained on the database page, but most stuff is on Dulles or Sterling dispatch, and add Leesburg PD if that still isn't enough. You honestly probably won't have to worry about focusing only on your area. First, Lovettsville area is a little more rural, but Loudoun as a whole is surprisingly quiet despite it's population.