Yeah I was thnking about something along those lines, but then would need 6 templates for all merc units  

Oh, for that, I am not sure. One kinda cheesy way u could do it would be to have the unit promote into new template, u could call it  merc_inf_archer_a_pre.xml, which then features the upgrade capability to turn into merc_inf_archer_a.xml The downside is that the user wouldn't have a greyed out icon before they are able to pay the 10 metal.

Once they reach the experienced needed to promote to Advanced, I then want to have them "upgrade" to finish the process. Basically Merc dont recieved techs from forrge/wtf etc... they also dont rank up like regular "CS"  they are gready MFs!  They want more "gold" to fight harder

This is already possible, isn't? Can be done with characters like =P

You want it to promote upon upgrading? Or do you want the user to have the choice of upgrading by spending some metal vs promoting by letting the unit gain xp. If it is the latter, yes this should work.