I've been culling my fire reds for 3 generations now. I started with 10 very nice painted fire reds and one so-so male. The F1 I culled about 50% of the male, F2 was a bit less, now I think I will still cull 30% in F3.... all because of 1 bad male.
How big you ask? I cull any male that is close to sexual maturity (i.e. very small), for female, I cull at first berry. Reason being, if I miss a bad female, I might have to cull 50% of that clutch, if I miss a male, I might have to cull 100 shrimps in the next month or so.
As mixing them with rilies, I think that's a bad idea if you want to have higher grade PFRs. A few of my culls look just like rilies. Such as this one,
BTW, if I know how to catch all the shrimps in that tank, I'd just restart with all nice ones.