Size does matter, but spines are not particularly an issue. I had an ornate bicher that happily chowed down several juvie Synodontis eupterus (that I thought would be too spiny.) I actually saw one go down the hatch tail-first, squeaking as if calling for help. Generally speaking, if the fish doing the eating is sufficiently larger than the fry, no worries. It's more of a potential problem when the meal is close to the size limit of the predators mouth capacity. As for the bicher, all I can say is they're tougher than most fish. I suspect that even a predatory catfish would have let that eupterus go.
Incidentally, the same bicher also chowed down an adult convict. It was originally intended to be a fry disposal, but decided that the adults were fair game. Taught me never to underestimate bichers...
