The only thing about no oc's is the roles would be pretty small and it would be restrictive.
Allowing oc's would be more open and less restrictive. One sorta middle ground option is to allow oc's but only of characters that have been shown to be at the very least a background non speaking part, some kids shown at the end of the 6th movie, a scene in the lonely journey, various flyers shown flying around in the sky, other characters including adults shown in background shots but no speaking or given a few lines in this or that song, or a spoken line or a few lines here or there, ect.
I've mentioned some ideas about practicing and such but since I've said it before and it seems to be ignored I"ll not mention again.