I personally really like the SF RCs in the realms I have. I think all my characters lead SF units. They are very effective and do not die as quickly.
In Melhed where SFs are likely used more than in most other realms, I think we have 7-8 RCs, the CS of the SF troops is 6485 of the total 15815, 41% of the CS. It's 293 of 1067 men or 27% of the men. That's from 6 units. I can understand that this is more than you want in an army.
The RCs that I use are in my region, so I will likely just have to draft more and deal with the concequences.
I would say that restricting how many you can recruit based on your honour and prestige would accomplish what you want better than this would. Drop the number of men you get per honour by half? Maybe restrict it to cavaliers and heroes? Or create a new class, but that seems like alot more work than it would be worth, and you would likely want more reason for it than just to command SF units.
Dropping it from the 100 for a lvl 3 is the right move though, I guess figuring out how much lower is the challenge.
My point is that I do not think this solves the problem you have highlighted.
