but like all living things, companionship from the same specis in critical for healthy development. um.. no. sharks, rays, most eels, angels, as well as a plethora of terrestrial animals.. not to mention plants, fungi, bacteria, and protists.. but that's rather anal
As for the spines, it's the dorsal spines, (top of the body) that are poisionous. as well as the pectoral, beware them as well
Like a bee sting but only 10 times worse. So, don't go in a clean the tank without partitioning 1/2 of it off first. Then go for the 2nd half.
my lion stays well away from me when my hands are in the tank, i have heard the same from others, such precautions are generally unecessary