You can do this, but you would create a Validation Group and in the 'Applies To:' drop down select faction then one of the non-outcast factions. Add the tag to restrict mercenaries to 2, then just copy this validation group and change the faction to one of the other non-outcast factions, repeat for each non-outcast faction. Probably need to make an enhancement to allow selection of more than a single faction here.
Does it make any sense to be able to add and/or edit non-label Tags on the main Tag screen?
Possibly. The reason that only label tags show up now is they are currently the only ones that can be applied to multiple Item Defs/Items. Since all the other tags apply to specific ItemTypes/Items I thought it would be best to keep the grouped with their relevant ItemTypes/Items.
I tried the validation group but can't get it to work. I think the problem is that the mercenary models are still Outcast so they don't think the rule applies to them. For example, a Guild army shows Guild models and any Outcast models that have "Include in Faction". (These Outcasts also have a Label tag for Mercenary.) The validation rule has "Applies To: Guild" and a tag with max 2, "Applies To: With Label Tag: Mercenary" and "Applies To: With Label Tag: Minion" (in an "And" group). The mercenaries are still considered Outcast models and ignore the rule. If I run the rule with "Applies To: All", it works correctly in any army other than Outcast. Maybe it is checking the model's faction and not the army's faction.
As to the Tags, my memory failed me. I was thinking of the Stats Type options that allow Text or Text with Heading. I was thinking of something were I could have an ability name and its rule. Of course that would not be appropriate as a Stat since not everyone would have the same abilities. I can use a regular Label tag for the ability names and a separate reference sheet for the rules.

Thanks
Justicator