You can get skill on any mob, but you can only effectively do skill up parties on certain types of mobs. Namely, you avoid mobs with dmg, nasty aoe, and mobs in bad areas with lots of aggro. An additional requirement is usually a mob that is EP or DC to lvl 75s, as these are prime for skilling from 150-230. This shouldn't be such a strange concept.
Not sure why you are even singling me out, Padington was the one that said he was skilling up, and I was refering to all mobs that might be used for skill up by saying 'skill up mob'. Its the same thing as refering to crawlers in crawlers nest as exp mobs. Who said anything about it being a rule, unwritten or otherwise? If you want to start an argument/debate with me, find better material.