I'm trying to accomplish something very similar. In my case, I would like to make it so lords stop recruiting trash non-culture troops. I've found that I have to manually remove these units from them every day, so as to avoid them getting bloated down with lots of generic units that lack cohesion with the main army.
It seems that with every major battle my lords will recruit hundreds of different unit types which makes it a nightmare for both logistics and party tooltip bloat.
Tweak 17 i) Appears to have functions to deal with this. I haven't implemented or tested it myself but it looks like we could simply lower the relation modifier for AI to get the desired effect. Probably not a perfect solution but it's all I could find.
It might also be worth mentioning that there exists a blacklist function that is currently only applied to Noldor troops. (Tweak 17 h) With that it might be possible to inject other troop indexes into the formula so it applies to other faction troops.
Edit:
Maybe you could scrap all that. See (Tweak 15 i) This relates to prisoner recruiting based on culture.