right... but wouldn't be cutting off it's tail. no seriously though, pygos may go for a ray. they nibble and try everything, garbage collectors.
but the elongs a hunter. ones ive kept/keeping, from observation, temperment wise -reasonable chance may not. the way the ray's wings move is disimilar to flicking tail of a fish, ray's size/shape, bottom of tank. i think it's the same logic as why armatus and atf's can be kept in a tank with rays.
as for a brycon, fast moving, shape/color, elong is hardwired for this kind of prey.
ofcourse this is pure speculation, if i find a small ray on sale may try this as a cohab while the ray is growing out. got plenty of spare room to move it.