I heard that as well. It seemed that at tourney’s people were stacking their armies with all hero’s so a general house rule of one hero. I do not think it’s mentioned in the rules at all. Some tactics can be employed as far as 4/4’s go. I too have no problem mixing units.
example: 4/4 Bar gunners support 5/5 Ranger’s I sure people can come up with more but that just springs to mind.
You have to be carefule I was used like a tool when I sent KAR 98’s against Screaming Eagle’s (First game) It was a blood bath. The trick is to play to the utits strengths and use supporting units. I’m a big fan of snipers and MG42’s