With Prometheus barrels I find Prometheus rubbers to work best, maybe because Prometheus barrels have a larger outer diametre than others. Also, many barrel spacers will not fit over a Prometheus barrel. I know Systema and VFC spacers work.
As for barrel length just stick to 509 with M16s, any longer and it will stick out. The only gun I know of where 550 is the perfect length is the CA/G&G M14.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN