A better solution to this problem would, in my opinion, simply be to further increase the Honor to Recuitment Cap ratio. If at 150 honor (which takes considerable time to amass for most people) I can still only recruit 25 Cav/SF, most people aren't going to bother to recruit them at all, making them the playthings of the very honorable and Cavaliers/Heroes, and those class options come with major drawbacks (like losing all your region maintenance/looting buttons and that non-trivial chance of death in battle).
Since there already seems to have been a decision though, I would add the following:
Particularly for cavalry, if their numbers are going to be restricted then they need to be better. SF seem to have artificially higher median equipment and training values (rare though not impossible to see any stat under 70%), but cavalry span the full range from useless (25%) to awe-inspiring (100%). This change will make low-end cavalry entirely useless. Why would you want cheap cavalry when your center is going to take forever to fill up anyway? They're only good in numbers, so really they're not good at all anymore.
My advice would be to bump up the average equipment/training values on cavalry centers. Maybe not as high as SF, but higher than they are. After all, you can't just draft a peasant and stick him on a warhorse. Cavalry require specialized training and were historically either born to the saddle, as in many central asian peoples, or were professional soldiers, and you didn't arm your cavalry with pitchforks.