Are you quoting Lean's Engine Reporter?
I don't have copies by me, but I recall that Poldory did have a 90 for a time. There was a period (I forget when) where quite a few of the 90s on united/consols were replaced with 85s. To the point that Consols started off with several 90s and ended up with only one (Woolfs). I think the only engine on Consols which wasn't an 85 or 90 was Pearces (at that time). I think United had 2 90s, one being on Poldory and one being on Cardozos, then it went to 3 85's, one being on Taylors, another not on cardozos and the other in poldory. I also thought that as you say, Loam's Shaft was one of the poldory engines and the other one may not have been. Old Engine shaft perhaps?
The little engine is in the reporter doing some ridiculous work. It was only pumping from adit, which is about 80m there, so not much pitwork in movement. I assume it was probably balanced from surface with probably only one plunger lift sat in a sump. I don't think there is any record, but I don't imagine it was running big bore pitwork either.
There was an engine on poldice (which is in lean's reporter, but not in "On the steam engines of cornwall" or whatever the DBB book was called). Which was a big one (80+) and did something like 12 or 14 spm. Trussals, I think. It's not one of the well known poldice engines.
I was thumbing through the original bound copies of Lean's Engine Reporter in the studies library. IIRC it starts off at 1812. It is a fascinating document.
I'm not 100% sure. I'm always keen on finding more information about the area as it is a fascinating place. The Gwennap book (old one) is brilliant. That's also in the studies library