This is an issue that is pretty difficult to solve unless you have determined a conclusive way that you are going to rationalize the abstarction of hit points.
Once you have an idea on that, then you have a starting point with which you can measure the difference between minions and non-minions.
For example, say you've taken the approach that Hit Points are a representation of the cumulative threat of death (or defeat) to a character... that each action taken by the enemy is driving the character closer and closer to the point where they can no longer avoid that final, deadly blow. In this scenario, you could assume that the difference between minions and non-minions is their general ability to avoid what would otherwise be a fatal wound.
In this example, lets say you have an orc taking a swing with a big nasty two handed sword that is aimed to take his enemies head off. If his enemy is a trained fighter, he may bring his sword up just in time to block the swing, but the force of it jars his shoulder and throws him off balance... not inflicting any lasting physical injury, but increasing the threat of the fighter's ultimate death if he cannot find a way to turn the combat in his favor.
If that enemy were instead a minion, he would not have the training (or strength, reflexes, or pure luck... however you want to look at it) to block the blow in time and would end up headless on the floor.
If instead you want to say that the loss of hit points represent actual phsyical wounds, then a trained fighter might be able to turn just enough that a dagger aimed for his gut merely cuts his flesh, whereas a minion would be caught unawares and takes a blade to some vital organ.
Once you have a solid idea of how to represent hit points, it shouldnt be too hard to put everything else into place from there.
Edit: Tomtill beat me to the point regarding the abstraction of hit points and to hit rolls, and did it better than I could I might add!