Mostly this mod is intended to have the most playable, bug free fun of any companion mod, with regard to fighting and staying with the player.
Buy the merc(s) you want from merchant Esther in the Imperial City Aboretum. If a merc dies, return to Esther to buy a resurrect. When you purchase a merc, a contract is provided.
If you don't have the contract in your inventory the merc will not fight, follow, or even wander around, he will stay put. Also, if the contract is in your inventory and you activate "Esther's Mercenaries" quest, the merc for that contract will be indicated on your compass and map.
There are several merc(s) that you can buy: Dog, Wolf, Bear, Barbarian, Archer, Mage.