And where do you get this? I don't recall any dialogue about the Stormcloaks hiring sellswords. A lot of the officers are Great War veterans. If you listen to the pvssyr in the camps, none of them talk about just wanting to fight. They mention not being willing to let an elf tell them who to worship, that a relative was kidnapped by Thalmor, and "I'm a true Nord, that's all there is to it."
Because before the civil war the Stormcloaks were a private army made by Ulfric that was mostly Mercenaries that would do mercenary work.
Well, to be fair many of empire legionaries are rookies too, with no experience since many died in the great war if I remember right at least two legions were completely destroyed
Yes but the legion has better weapons, better armor, more men, better trainers etc etc, stormcloaks are stuck using only iron and wearing only fur.