Why wouldn't the Simians seek shelter among the Hivers?
Ran,
Let's see...
The Simians would be escaping from the Solomani, the race who uplifted them to sentience and nearly exterminated them out of hand as a failed lab experiments, to live among the Hivers, a race who manipulate every minor race in their Federation so that they can better fit into whatever sociological box the Hivers have chosen for them.
Sounds better to me.

Imagine you're a lab rat. Does it matter if the fellow in the white coat has two arms or six tentacles?
It seems they would desire a buffer between themselves and their oppressors...
Trading one master for another? Sure, why not?
To the Hivers, they would be just one more Terran primate living inside their borders.
Yup, just another Terran primate living inside their borders that any starfish with an M in front of their name or who wants an M in front of their name can play with.
This way too, there is no Simian homeworld subject to potential pogroms by genetic purists, abuses by slavers and bounty hunters, or even squatters looking to jump claim on a colony site.
You really need to read about the Hivers, really.
IMHO, the Hivers offer them something the Solomani don't, a place at the table.
A place at the kiddie table.
Escaping to the Wild Wild Rim of the Confederation is easier and can potentially give the Simians control over their own destiny.
First, while they were nearly exterminated and now are studiously ignored, S&A doesn't even hint at pogroms, bounty hunters, or any of that. The best way to describe official Sollie policy towards the Simians is "benign neglect". Second, the Simians already have Sollie allies. The reform movement in particular is striving to make the truth behind the Gene Wars known and has enlisted the Simians themselves in that effort.
Because the Simians are already part of Sollie society, albeit on the margins, and because they already have sympathizers within the Confederation, "escaping" to a safer place within the Confederation is much more easier and can produce greater results than starting from "zero" within the Heirate, Imperium, or Federation.
Regards,
Bill