There's plenty of ways to improve guards without indirectly buffing Kroak...
And even if it would (indirectly) buff Kroak it wouldn't necesarly be problematic. There's already some nice limitations keeping the combo in check to some extend anyway. The combo is reasonably expensive, the bodycount is low, they're slow, they can't leave eachothers side...
The only buff that might be problematic is giving them more wounds or a wardsave (though an obvious solution would be "ward saves don't count for selfless protectors"). But something like an extra attack, a better save, the ability to do something besides being ablative wounds for a slann all shouldn't really impact the Slann + guard combo...
As for protecting skink & saurus heroes on foot. All that this achieves is that especially the skinks can no longer be sniped by anyone who looks at them funny by essentially doubling the prize of your skink. That doesn't exactly sound like an ridiculous trade-off... It's not like 5 guards + a skink priest are going to make an unstoppable force, nearly everything of (roughly) equal value will win in a straight up fight...
Any kind of buff to kroak would be flagrantly offensive to the people that play this game.
Literally anything you add to guard is just a straight buff to an already good unit. An extra attack or better save? Cool, now that battleline you were taking anyways to save Kroak can also go toe to toe against some of the tougher stuff in the game. So not only is it giving extra wounds to your godly spellcaster, it's also doing decent damage on its own. That sounds fair.
Bodyguard for skink and saurus heroes? Basically our only weakness on those heroes is their relatively fragility. Carnosaurs do disgusting damage for their points. A carnosaur with 15 guard around it (the necessary battleline you were taking anyways) suddenly has 30 EXTRA WOUNDS. That's so strong it's insane.
Those weak skink heroes that so much of our army relies on? Suddenly unkillable. Making guard better literally makes the only weakness the book has irrelevant.
Put 5 guard in your little castle and it essentially makes all your heroes immune to everything but getting straight up charged and if thats happening you've probably lost anyways.
Think about it this way. Would you spend 310 points for a carnosaur with 10 extra wounds? Yeah. And that doesnt even take into account the guard can hold objectives on their own, also catch wounds for your slann/kroak and have 10 attacks of their own. Strong doesn't even begin to do that justice.
Even if you say its only foot heroes, that astrolith is now unkillable, and you still have the skink problem. The skink heroes (priests and starpriests especially) buffs are simply too strong to have such a strong safety net.
They don't need a buff. They have a role. They do it well. If you make them a stronger fighter, you'd have to nerf their bodyguard save. If you make their already incredible bodyguard role better, you'd need to nerf their individual strength.