10-12" CC from 4-6" stockers in (less than) one season is definitely possible with limited feeding, depending on location. What's your 20?
OTOH a CC reaching 18" on once-a-week feeding would really be pushing it, and I don't believe a 6" LMB could grow anywhere near 2-3 lbs in this amount of time. If there is a chance that a "helpful" friend, neighbor, in-law, poacher, etc. could have added large fish to the pond, I bet that is what happened with the large unidentified fish and the 18" CC.