getting back to the original question...and yes, some more explanation should have been given other than just "no" in this case:
clown knife of similar size could work, but this also depends on the personality of the CK...some are more aggressive than others.
the shark is not a good idea because they are actively swimming and silver...both are hard for a gar to ignore, and since it doesn't have any scales, the shark could get beaten up quite easily and/or killed. the shark could also spook the gar into breaking its back.
so i would say a conditional yes with the CK, no on the IR shark. good luck--
--solomon