Anything bigger than about 6 thousandths of mouth should be fine. Personally, I'd probably file the mouth 6 thousandths if I had an iron that was just dead on fitting tight, but before you do that to a beautiful plane, you want to know that you can do it accurately.
I'd use the tapered iron that came with the plane, it's a beautiful iron, and the only reason I'd replace it with something else would be if it's soft or chippy. There are a lot of infills with tapered irons in them, but many quite a bit thinner at the top end than that one (and thus funny looking). The fact that that one has a make that matches the plane and is near parallel would lead me to use it.
I have noticed with the dozen or so infill planes that I've used (pre-war, before norris started using robert sorby, because all bets were off at that point), that the older parallel irons are generally a click or two harder than most of the tapered irons that I've used. I. Sorby's parallel irons will be about as hard as mathiesons, will be about as hard as wards, etc. Robt sorby's irons are perfectly serviceable, but they're not made to the same standard. If the taper iron that came with that plane is reasonably hard (you can always get a good judgement on hardness by taking only thin shavings for a while - an iron that is soft or chippy will drive you nuts in terms of sharpening cycle with thin shavings), I think it's fine as it is.
Anyway, if the taper iron is dead tight and you want a .006" mouth or so, you want an iron that is at least .006*(2^0.5) = .0085" thinner than that iron is right at the bevel (the taper is little enough that you can measure it there). Certainly, the old irons vary in thickness a lot, even in the same make, so I guess you'd have to check with a maker to make sure you got what you want.
(I wish I'd have gotten that plane for 140 quid. i'd file the mouth a little bit and nobody would ever be able to tell that I did. If I was absolutely dead set against filing the mouth, I'm sure I could get 8 thousandths off of that iron with my lap pretty easily. Might take a half hour of sweating, but doing the filing job properly won't be 2 minutes - it won't be an hour, though, either).
