skill wise, i know that at least some of the basic HtT mercs like ghouls and guys like the white metal armor wearing sledge hammer merc can use both melee weapons, and small guns reasonabley well, i gave them both small guns and they do enough damage to be worth it, even if they don't burst very often if at all, not to mention both mercs mentioned have 100+ HPs so its basicly a tank that can attack stuff following you, even if not kill it quickly.
thats all i know thus far, hope that helps you to guess what they can do.
-Ulrek-