hehehe... oh I figured they were not even remote possibilities, but I've found that while discounting the impossible, sometimes it inspires.
Personally, if someone asked me for a Proto-Sliver familiar, I'd say "Go to the Queen and get one yourself, ya bum!" or something with slightly more...
Just read the template. It seems good to me, I think that what may be throwing you off is that, by itself, it isn't much to look at, but when that poor Half-Sliver Fighter gets some of his Sliver pals together, well, then he's a force to reckon with.
Perhaps something along the lines of a...
Not entirely satisfied? What seems to be wrong with it? Not powerful enough? TOO powerful? Not "Slivery" enough?
Or just shuddering at the thought of an Axiomatic Half-Sliver Bulette?
Post what you've got, we'll be gentle, and I'm sure we'll all help you out.
Wonderful job on the Overlord, and well worth the wait. Hell, it would be worth two waits for a monstrous abomination such as she.
Woe be unto the being who stands before the Overlord...
Excellent job, once again.
You'd probably want something a little closer to Dominate Monster, though as you said only against Slivers. Maybe make it some kind of radius? How's about this:
Dominate Sliver (Su): Every Sliver within 60' of the Sliver Overlord must make a Willpower Save (DC whatever), with a -4...
The Cerebrates of the Zerg each controlled their own Swarm via massive telepathy, so in that respect, the Sliver Queen could almost be likened to a Cerebrate more. The Noble Slivers would be like your hero units (Torrasque, Devourer's Child, etc.)
Now, the Overmind controls all the Cerebrates...
I would imagine that, since each Sliver has an Intelligence score, that they aren't actually drones in any regard. However, since every Sliver exists to ensure the survival of the species and to protect and serve the Queen, it may seem this way.
The Queen is intelligent enough to be her own...
Magnificent work! The sample swarms are very helpful, and indeed are pretty much the "nail in the coffin" to any party of would-be heroes. Goodie Two Shoes heroes... bah. It's their own fault for picking on the poor innocent little slivers, now isn't it?
Eagerly and patiently awaiting the...