I'm no expert here, but it does intuitively seem to me that this is a situation that SSH is not quite programmed to recognize, viz: "even though the IP-addresses of these 10 machines are (of course ...) not the same, functionally they are 'a band of brothers.' "
Therefore, I think that I would, indeed, issue one common SSH host-key and associate it with all 10 of these addresses. Because, well, that best reflects the reality of this situation. All 10 of these IP's (but, none others!) are supposed to share the same credential . . .\\
(Any objections, dear experts?)