Will be a difficult mix as the eel will want much rock work and many nooks and crannies whereas the flounder will want much open Sandy bottom space to move around on and bury in. Not sure how you will achieve both without a very big tank.
I have kept snowflake eel with many other fish successfully but never tried flounder simply because of the above. Had snowflake, bat fish, porcupine puffer, polka dot grouper and damsels in a 6x2x2 for a number of years but not tried tile and flounder.