I have the exact same concerns! We'll be doing Fantasmic this Sunday as well. Although I've been to DL multiple times, this is the first Fantasmic for me. The best advice I've seen is, there are good spots in every FP area, and bad spots. Just be there on time, and try your best for a good one.
If you see us, say hi.

We're 3 generations of brunettes, 2 adults and a 13 year-old, headed to DL for my mom's 60th b-day. We'll be wearing matching shirts as well.
ETA: I just re-read your first post. I see DCA for only one day. Did you get one park per day tickets? Many of us choose parkhoppers for DLR since they are right across from each other. My group usually hits both parks each day. DCA is a favorite of ours.