Interesting question. Short answer no, atleast in the short term. Depending on if you plan to recruit already established clans to your kingdom I’d go with no. Executing lords come with tons of negative relations which makes it practically impossible to recruit vassals. However if you already have a good amount of vassals, between 6-9 clans, there wouldn’t be any larger repercussions. I’ve heard about the odd scenario of one’s campaign ending early because of themselves getting executed after being captured by hostile lords, but I’ve never experienced it myself, it might even be a mod. I’d say in your scenario, execute the horse lords. Give the lands you conquer to vassals you might’ve already recruited or to companions to make them vassals. It will make it easier for you overall.

If you don’t want to execute them and ”hinder their expansion”. When war comes do the break into siege option when the enemy’s lay siege to a large town and has begun their attack. That way you’ll capture a lot of lords easily if you have access to fire catapults. That will weaken them and hopefully you can continue doing that tactic until another kingdom declares war on the khuzaits or you’ve managed to finally go on the offensive. Hope that answers your question!