Why do they all have to go into a single class? Because if the Runoffs is "just another weekend race" with all the wings-n-things thrown together into a single race group what justifies driving so far and spending so much time, energy, and money? The Runoffs has to offer something special to attract the serious competitors in the classes, and single-class races have been the ticket to that for decades.
Ignoring this year's FX race, the last time I recall the Club combining two formula classes in the same race at the Runoffs was the 2004 FA race, which featured FE cars packed into FA to give them a place at the Runoffs. On lap 13 of 20 Dan Cobb, FA pole sitter and race leader had contact with Keith Grant's FE while lapping Grant, and both were out of the race.
The root of the problem was that the FEs were 10-20 seconds a lap slower than the FAs. If the cars are closely matched, like say FA, FB, F3, and PFM, and there are a reasonable number of entries (<30?), then it can work and be enjoyable to watch. But as this year's VIR majors race taught us, you can't throw 69 cars on track together and not expect a lot of carnage.
This is just discussion and not an answer, so don't read too much into it.
Disclaimer: at the F/SR tent meeting this year I did suggest P1 & P2 could be combined to produce a a good race, and along the same lines FA & FC could race together. Again, it's just a conversation starter. YMMV.