you know, i think this is pretty spot-on even without R63 , in the story they wrote more than once that he's delicate and that you could mistake him for a mare. (just read the fic in 1 go, so the memory is still pretty fresh)
Because one male can impregnate any number of females without difficulty, but once a female gets pregnant, she's no further use for breeding for an entire year. I'm not sure each mare would be able to produce enough stallions to keep up over the course of her, ahem, "service lifetime".
They need one body for every corpse. Doesn't seem too unreasonable. I mean, of course it is unreasonable, but it isn't impossible and doesn't really seem infeasible at all. The bucks were only used for offspring when a member of 99 died. Why couldn't an inverse stable keep up?
If PH was R-63d, Stable 99 would seem like Flank....
and Flank would seem like Stable 99...
Huh. Weird.