Do you use your head before you comment?
The Metamoran and Potara are completely different.
Chapter: 469 (DBZ 275), P6.1-7
Context: after Goku laments that Gohan and Vegeta are dead
Goku: “I coulda used Fusion…”
Dende: “Fusion…! Merging together, right? That’s the specialty art of the people of Planet Metamor!”
Goku: “So you know about it, Dende…! That’s right, some people from Metamor who I met in the afterlife taught me that art…It’s a merging technique which can only be performed if two people are fairly close in both power and body size…In other words, by having two people merge into one, they’re able to become a single, new human with amazing power which either of them on their own absolutely wouldn’t be capable of. It really is incredible! Those two from Metamor were completely weak and gentle on their own, but by using Fusion they transformed into a substantial warrior! [ ] …I was just taught the art, but I ain’t never tested it out…There wasn’t anybody on par with me in the afterlife…”[/quote]
Goten & Trunks tried to merge as Super Saiyans but they couldn't control their Chi properly, it was too hard for them Pre-RoSaT. They countered this by being able to master the Metamoran and transform AFTER fusing.
Goku wasn't aware of this since he didn't suggest it in the first place and asked them to fuse as Super Saiyans from the beginning. So the theory of fusing in his Base form is contradicted already.
Goku says that to perform fusion dance the two fusees must be "fairly close in power". You can see that that's slightly different to the "exactly the same power" they need when actually fusing.
So for two people to fuse, they need to be close in power, and once that requirement is met, they need to match their Ki to be exactly the same as each other. This is further backed up by Goku saying he couldn't fuse with anyone because there was no one on par with him in the afterlife, despite Goku being able to suppress. Since Goku planned to fuse with Gohan via the fusion dance, that would make Ultimate Gohan "fairly close in power" to at least some form of Goku, likely SSj3.
All evidence suggests Goku would perform fusion as a SSj3 because he wasn't/isn't aware of transforming into Super Saiyan after fusing, and suppressing battle power wasn't relevant enough for Goku to fuse on Planet Metamor, because the two product of the sum need to be CLOSE in power to begin with BEFORE matching their Ki.
The Potara has requirements that need to be precisely followed and the Potara doesn't.