They never competed at the same weight (at world level). Khaosai would give a good account of himself at the very least, against a younger, drained Fenech. But there are way too many 'what ifs'. So Fenech is the obvious pick.
At JBW, Galaxy. Fenech would be too drained and weak at that weight. At BW, Fenech. Galaxy never fought there, although the weight wouldn't have been an issue for him...he probably could have fought comfortably at featherweight with such a stocky build. He'd be giving up height and reach and wouldn't be a "weight bully" at BW, and I don't see him beating Fenech.