I dont think skirmishers and cohorts are separate models.. I havent ever seen any different boxes than the standard skink box that contains 24 skinks 6 skinks per sprue with 5 of each weapon choice and i believe 6 shields aswell (including some extra bits).
Anyways, i use the same models for both tasks and i find my cohorts extremely usefull. they are cheap and functional against the right opponents, add a kroxigor or two and they are good against pretty much anything, except first-striking units.. High elves..
I almost allways bring one unit of 10 skink cohorts because they can do what i would want swarms to do, move quickly, block sight etc etc. Its a unit to be creative with.