ANY of the Apple 800K drives will work fine on a Mac.
Plugging the UniDisk into the Mac will not harm either. It is compatible if you open it up, unplug the "smart" board from the drive and connect the external cable connector directly into the drive header.
(See this for more info http://68kmla.org/forums/viewtopic.php?p=44706#44706)
You probably have the Apple 3.5" Drive in any event.
FYI, there is nothing you could do with a 5.25" drive connected to the Mac FDD port anyway, even if it didn't fry your system.