Comments/Ratings for a Single Item

Cannot be captured until it has captured a piece

This reads as if it loses its protection permanently upon capturing, whereas Spy Chess' eponymous piece is vulnerable only in the turn after a capture. Seems to me the latter must be what you mean, especially as the former doesn't look (at first glance) like it'll confer enough of an advantage to balance the FIDEs (and of course the reference to Spy Ch. is very suggestive), but it'd have to be expressed more clearly if so.

If, on the other hand, you do mean that it loses its invulnerability permanently, it'd be worth stating the distinction from S.C. explicitly.