the skill system is very similar to the way SWG was in the good old days.
I was out mining awhile ago and got aggro from some mobs. The last combat class I was leveling was Conjurer(last night), but I switched out my pickaxe for my war axe(Marauder weapon). My action bar was still loaded with all my conjurer actions, and none of my marauder actions. Needless to say, this actually worked out really well for me, as I was able to keep myself cured and cast some pretty decent damage on the mobs:)
Mixing combat class skills is something I really missed from the old SWG. I'm glad I have that again in FFXIV :)