Should be doable. I'de settle the eel in first get it hand-feeding then intro the clowns. once the clowns start eating they will eat everything, so unless your overfeeding the tank the eel might not get enough and the clowns will chow down on anything you'de feed your eel. Also plan on having a big tank to house all these monsters together. And have more hiding spots then fish in the tank even though the clowns will likely hang out together. I know my eel is very territorial of his hidey spot. and clowns can be ime. I have never kept the 2 together though. my fire eel at 14"+ still won't touch the rosy minnows i feed his tank mate, a needle fish. kinda funny watching him go nose to nose with them. so I wouldn't worry so much about size differences. but it is possible the eel might try one if he's introduced last and not eating well right off the bat.