HK93 is a civilianized version of the police and military HK33. The 93 has a shelf trigger pack (not a push-pin like on a 33) and is not full auto. Not to mention no flapper mag release and a bunch of other differences.
The main differences between an HK93 and HK91 (a civilianized G3) are the caliber and size of the rifles. The 93 is slightly scaled down to deal with the smaller cartridge. Many of the parts interchange between them.
The Sig 550, FNC, FN-FAL are all different rifles. The FNC is an updated version of the .308 FAL, in .223. The Sig 550 bears little to no relation to any of the above.
The CETME was the precursor to the G3. It was designed just as WWII was closing (you can see it's lineage in the StG.44, the first real assault rifle, which was fielded by the Germans before the close of WWII)...the Spanish Army needed a rifle, and an ex-German small arms engineer came up with the design that would eventually become the G3, based on all the previous work done at the close of WWII.