Is It Possible To Balance Custom Mobs For Low AND High Level Characters?

Simple question, but I'm looking for tips here from people. The problem I have with my cow arc is this; I've got 3 custom mobs for the cow faction, with synchronizing powers between them... the idea being that each one is a weak link that anyone should be able to tackle with an opposing power set, but until 1 is knocked out the herd in general remains strong. They are all Lieutenants, because at that level you get 1 of each in every spawn.

Pre Issue 16, as they all had variants on Ice powers (Cold Milk/Ice Cream to personal taste!) but I'd taken some of the debuff powers off each one due to stacking -Recharge, and spent ages balancing them out with experience from my own different alts and the copious initial MA excitement of others who'd played it, so they'd be tough but fair... they then seemed to scale from there nicely for everyone. But I16 seems to have thrown that all out of the window now though; I've tweaked and tweaked again but the problem seems to be that with less powers than normal, they are ungodly strong against lowbie characters due to ramping up to compensate for this, and to prevent easy farming; but with Standard/Standard they're still rather tough against Low to mid, but turn out incredibly weak against L50s.

At present in my arc, I can beat them with both my L30 Peacebringer and my L50 blaster... but my first play, and feedback in nearly 3 months from a L29 Claw/Inv brute tells me he can't beat the final spawn at all, so I'm close to tearing my body hair out trying to understand how to get the balance right. How has everyone tackled this problem on their maps?

Also if there's a work around to force a mob to consist of at least 1 of each, so I can reduce some of them down to Minion instead, do let me know!