I'm a sci-fi builder more than other types of kits, and I don't really see the need for a sci-fi only issue.
Maybe a subject here and there, but not a whole regular issue. Perhaps a special issue, outside of the subscription that the subscribers, who are mostly military and aviation builders don't have to receive if they don't want it.
Besides that, I would be worried that some sci-fi builders would rip it apart anyway if their specific tastes were not included to their satisfaction. Too many Trek kits would PO the Star Wars builders. Too many Alien kits would PO someone else. I remember when the Guide to Star Wars ships book came out, people tore it apart saying that it was too simple, and the kit choises were not to their liking.
FSM has enough information that can be applied to all genre of kit building. There are already plenty of Internet sites with Sci-Fi kit reviews and build-ups with plenty of pictures. I think an issue of FSM devoted to Sci-Fi would seem like recycled information.