No, adr at 6:30 at Brown Derby will NOT give you enough time...that's a signature restaurant. You can count on 75-90 minutes even if seated right on time (which probably won't happen, either.) Even if "only" takes 75 minutes to eat there, by the time you get out & get over to theater it will be 8:00 or after. We have been at Fantasmic in September several times and though the crowds are lower in general, they really cut back on the Fantasmic performances so everyone crams into the few shows they offer each week...we've found September F! crowds to be more crowded than May, even.
I would not schedule an ADR past about 5:30, maybe 5:45 if I wanted to get to Fantasmic that same night.You really want to be at the theater no later than 45-60 minutes before show time (and you will not have a choice of seats, but there should be a few rows left here & there. Good view all over, as long as you're not in the standing room only sections.)