I figured right away, for common sense this is a big no-no to do against lower levels, but against a group which is higher level than your own, and actually -has- a chacne to spot you, be it on their own or through magic -

Is it acceptable to steal from a group as they lay down loot, from stealthmode? Obviously something like a large weapon or armor could not be taken and just vanish into thin air-

But something like a potion, or maybe a few coins - Would this be allowed?

Yes, there is nothing wrong with this in any way. In an NPC area, the PVP guidelines do apply though (get a DM).