Green stuff's definitely at it's best when it's mixed with another putty.
Some procreate users (including me) find that the claims of a hard cure don't quite work out in practise. In my experience it's a little harder than green stuff, and brown stuff's harder still. But to be honest, while they have their uses (keep bugging the shop owner for PC), the fact they're harder than GS isn't saying much. They both still have a certain rubberiness to them. Mixing them with GS won't help that.
For rigid, easily-cleaned pressmoulding results, I'd head straight for the properly hard-curing claylike putties. They also have the advantage that when they're freshly-mixed, they're a lot softer than the elastic, blu-tack consistency of GS, BS, or PC. All the better for pushing and squeezing into pressmoulds.
Milliput's the hardest, in my experience, although it can be a little stiff to mix if it's not a fresh, new batch. I've started a very fresh pack of bog-standard yellow-grey (no traces of age discoloration or nuthin'), and it's like butter compared to my old pack of superfine-white. How much that might have to do with the different products, I dunno. Superfine-white also seems a little more elastic to me, even when it was fresh. But anyway, if you mix it with GS and pressmould it, you might not have to go with the most expensive, premium type of milliput.
It also seems to have a tendency to ignore or absorb lubricants (vaseline and cooking oil, at least) and stick, in my own pressmoulding efforts. Although the effect might not be as bad with a flexible mould material like siligum, than a table tennis ball cut in half...
And if you're using 50% milliput or more in the mix, it isn't going to give a monkey's uncle how much blue you put into the GS. It's going to cure hard.
Then there's stuff like apoxie sculpt, magic sculp and cold clay, which in my experience pretty much all act like the same thing. (At least, the differences are negligible compared to other putties) Freshly-mixed, they're softer and less messy than milliput. They don't cure just as hard, but are a touch easier to dry-sculpt. They also take notice of the sunflower oil coating the inside of half-a-ping-pong-ball.