I had one of those monster Peacock Mantis Shrimp with a tail less
Lion fish in a 20 gallon tank for a long time. No worries.
Used to feed then little feeder goldfish. The lionfish was a bit more aggressive so sometimes the Lion got 2 and the shrimp went hungry.
Before anybody asks --- The Lion fish lost it's tail due to a bacteria infection at the LFS. It healed but never grew the tail back. After seeing it in the store for many months I felt sorry for it and decided he could be the Mantis' friend.