I’m intrigued to try out a few different liquid yeast strains, but only make 2.5 gallon batches to allow for more frequent brew days. I’d like to avoid any easily avoidable issues that may come from overpitching, and unfortunately internet searches haven’t provided good results thus far. 
This may help: Pitch Rate - Wyeast Lab
If you brew fairly frequently and have a strain you consider a house yeast, just build a starter and save what you don’t need in the initial brew for the next batch. After the second beer, harvest some slurry for the next and repeat until you see any difference in performance.
When I make small batches I use an entire smack pack without a starter.
You’ll be fine using the whole pack.
Yep.
And don’t forget to save the slurry from the yeast that you like.
Denny is on the money. A 2.5 gallon batch does not require a starter when pitching a new liquid yeast culture that is not past its expiration date.
Same here. 3 gallons in the fermenter is my standard batch size and when I use liquid yeast I pitch one full Wyeast or White Labs pack. I’ve never had a problem with fermentation at this pitch rate.
Thanks everyone!! This helps a lot. Next steps … brew day. 
+1 - Also, overpitching is only really a problem when lots of dead cells are present.
Bingo! Overpitching is serious problem when serially repitching because it leads to insufficient new cell growth, leading to a culture that contains a lot of dead and old cells.