Haven't done any IS, or Merc units, but...
Once upon a time i entered
Clan Wolf as it appears in the Clan Wolf source book... Changes to MekHQ have since made the file invalid (though the devs seem to have been working on fixing it, and I've given them permission to distribute it) It's kind of a stress test for MekHQ, as when I made it the program would crash with memory errors after about 30 mins.