Hm. Well, let's see here. By RAW, compelling the pixies aspect of inhuman alertness against the players isn't legal.
Yet, compelling the scene aspect, "guarded by inhumanly alert pixies" against the PCs... is. That said, I usually wouldn't; there's a fine balance line between making aspects useful, while still letting skills be useful. As a PC with a stealth skill of one & no relevant aspects, I'd have no problems with taking my fate point and accepting failure without rolling. As a PC with a stealth skill of 3 to 5 and an aspect for being sneaky, though, I'd probably feel miffed about having to pay a fate point just to make my character function as I'd expect. And I'd similarly be miffed if there was a miscommunication and I had already rolled & gotten +4 on the dice and then had to pay a fate point to "keep" that result.