looks like a red belly piranha to me. Nice fish too. How big is he? If he's pretty big that may explain his indifference to the guppy. Big P's rarely go after little fish.
Like I said before I wouldn't do it, but if you don't mind taking a chance with your ID shark then go for it.
no id what kind of fish that is, but I doubt the id shark will survive long enough to grow out. You may have a better shot with the red devil, but red devils are nasty too, and may just kill the id as well.
I've tried jack dempseys, plecos, convicts, crayfish, oscars, tetras, just about anything you can think of that fit in his tank. My S. compressus kills everything. Most of the time the murder happens in the first 48 hours. One convict made it a month after losing all his fins and tail before he got it too. Just depends on the fish I guess.