I would say mixing those two shockers would be fine so long as you adjust the konis to a damping rate that is close to what the billies have.
Talk about tyres pressures always seems to pop up regularly. Lots of people have differing opinions on this it seems!
I have always thought as 16" and 17" tyres are mean't to have the same pressures (according to the manual) this would suggest that (as far as Alfa were concerned) difference in tyre profile between these two sizes makes only negligable difference in sidewall stiffness.
Also, as Alfa never specced 18" or above size rims with the GTV its hard to know what the pressure should be for those size wheels and tyres.
Perhaps owners of other Alfas (ones that did have 18" optional wheels) can comment if this varies greatly at all.
On my fiat (which is a lighter car at around 1040Kg kerb weight compared with around 1370Kg for the Alfa) the tyres are a little thinnner with higher sidewall 195/55/16 compared to my old Alfa's which were 205/50/16 but have way less recommended pressure at around 2.2 bar. Of course that car can't go anywhere near the speeds the Alfa did so my guess is performance and weight have the biggest bearing on setting recommended pressures, then tyre size and profile must factor in after that. That's at least 4 factors that have a bearing right there i'm sure there are many other factors such as temperatures,humidity etc that are taken into account...
To err is human, but you need an Alfa to really screw things up!
The road to heaven is paved with old Alfa's.