How does the Borg deal with non bipedal sentient lifeforms? Are there any who have been assimilated into the Borg Collective?
Other than the Species 8472 comment I made, I've found 2 candidates and one interesting note:
The Greech are this thing:
In the Voyager novel Unworthy, they, along with the Neyser (humanoid), the Dulaph (sphere-shaped silicon-based species), the Irks (a bio-mimetic species), a moth-like species, and a noncorporeal species, created the Indign cooperative in an attempt to mimic the Borg, when the Borg considered them unworthy for assimilation.
So in the novels at least, it seems possible (although unlikely) for non-humanoids to be assimilated. However, given that the Borg Queen is humanoid, it could well be that the Borg idea of perfection includes humanoids. Their attempted assimilation of Species 8472 would then be explained as an attempt to gain knowledge of their biology in order to adapt it to the humanoid shape.
So, No, the Borg are not limited to bipeds if "omnicordial lifeforms" are not bipeds - and even if they are, it is unlikely they are limited since they did at least try to assimilate the 3-legged Species 8472
Assimilation will require fitting into the Borg structure. I.e able to use regeneration alcoves etc, the assimilation process of non-bipedal life forms could just use the head and torso of the the individual assimilated and replace a quadrupedal or hexapedal limb system with a mechanical bi-pedal limb system. As is the case with the Borg Queen. Having not seen her original form, she may not have been a biped.