The new Mercenary feature, allowing you to add alt characters as heroes, allows you to wear anything you want. Costumes, newbie armor, hats, or nothing. The new heroes will still have full armor but they will be "invisible" in the slots you had no armor.
Just tested adding my two ritual girls as heroes to my monk. He's really no pimp but he deserved a bit fun with the girls after been inactive for a year.
You only need 1 slot, it's account wide. Sure you can only add one of your hero as a merc, but all your characters can use it. Buying 8 slots would be overkill I think. Personally I bought 3.
Mercs only copy the appearance while getting a new mercenary armor. They don't inherit weapons, skills, runes or insignias. You have to equip them from scratch like any regular hero. See picture:
I just did an experiment with a character I didn't plan to keep. I added a warrior as a merc, then I deleted the warrior. Now it remained as a merc hero and I could use it like it still existed.
So the mercs are truly independent of the characters they were created from, only limitations is that you have to be level 20 to create a merc, and you can't create a merc with same name as an existing one.
However, it would be possible to quickly level up a character 20 with a preferred class, put some newbie armor on and dye it, maybe some costume. Then merc, delete character and redo. Then you can easily get enough of the professions you prefer without actually having to have them as playable characters.
Maybe best to do with a PvP character slot.
ALSO.. I unlocked merc heroes on a level 16 character, so the only requirement is probably to reach Eye of the North. So it's possible you can get your mercs at level 10.
UPDATE #1:
Just created a new PvP character with same name as my old warrior, but different profession, campaign etc. My old merc with same name was still listed.. but since she used same name, she wasn't selectable.
UPDATE #2:
Just saw there is a Merc Registrar on Battle Island, I managed to get there at level 2 after creating a new Nightfall character. As soon as I talked with him, he unlocked all my merc characters. Now I could use 3x Lev 20 heroes at level 2 :O This gives Nightfall heroes a distinct advantage as they're can go very early to Battle Islands.
Having a bit fun with the new mercenaries, I created a PvP Equipment and Skill template, merced and recreated Borat a number of times. Take a look at the Borat Dream team lol
Micro hero skill slots to number pad keys and you now have uber SF spike.
Ontopic:
I am assuming those are PvP characters, can you use Merc Heroes based of PvP characters you made...?
Or is that only for PvP(assuming you are in GToB there)?
It's easy to create your dreamteam if you can consider PvP armor on them,
because they're accessible to your PvE character after. 7 necros? No problem.