Why not introduce something where when you have 3 of the same types of mercs hired (Super Elite, Golem, Mannequin, etc) you get a say 5-10% bonus on top of the stats they already contribute to your groups.
may not need to be a stat increase for them all, for example, you may get a 100% hypnotize enhancement by having a group of mercs hired.
Could make things a bit interesting and fun

