On the speed: Cheirolepis would have most likely been slower than a piranha but fast enough to outclass the sluggish but well-armored antiarchs on the sea floor. So yeah, 30 feet works dandy.
On the armor: It sounds about right, although I might suggest decreasing it to +3. +4 is encroaching on the small monstrous crab, which cheirolepis most certainly isn't. It did have a thick scale "plating", but it's not comparable to a chitonous outer shell or carapace. The closest modern fish I'd compare cheirolepis's skin to would be my native alligator gar; it's thick and tough, but it's not exactly a haulting shell.