I have never heard of a minimum fps require to scour gunk from a pipe.
Now, gunk from my ex's trunk - that requires a whole hell of a lot more than 2 fps.
Personally, if all you are working with is 13 gpm, I'd stay with SDR 21 1" pipe for everything, lateral and main and design/install to the lowest sustained volume and pressure. The safest bet is to assume the worst and install for it.
Nice Try = You Suck Spelled Differently