Could Greg have bonded to another another unit before the Matrix upgraded him to Prometheus?

At a guess -- likely no. Proto Guyver/Hybrid Guyver (I'd forgotten about the Hybrid name) is still considered a Guyver unit itself, when all is said and done. Probably the only way for Greg to have bonded with another unit was to have the Proto Unit removed from him via the Matrix, and then give him another unit. But you can't have multiple units bonded to the same host -- wouldn't be the least bit surprised if that was another function of the Limitation governor within the Guyver control medal.

I really depends on how the Hybrid Guyver works really. Is he some form of high powered zoanoid or unique zoalord? Or is he unique on his own.

Regardless, if the unit was capable of bonding to him, he wouldn't get that much of boost if at all. But he could at least bring himself back to his original power.

In the end he got the better deal anyway. Prometheus is plenty powerful. Destrol (unless his datafile is redone differently or he gets a boost) is half his power who is the most powerful in Canada. If not then he can tag team with Ultimus and maybe even Gigantic Zero.