Regarding the Turbo size I would go for a turbo off an engine the same size or one that is a bit bigger!
And I agree that the smaller turbo will spool up quicker but this will reduce top end power because you are putting out more exhaust gas flow than the turbo can handle!
This will lead to excess heat from the turbo and eventually turbo failure!

Regarding how much pressure to safely run in a standard carbie with out modifications to the seals! I have found that anymore than 10 psi causes the top seal on the carbie to loose boost! ( Anyone who has ever rebuilt a datsun 1200 carbie will know that the top of the main body warps and has to be filed flat! This is because of the flimsy design around the srew points!)
Because of this when boost is applied the main body flexes around the float bowl and cause boost to be lost! this is right neartheneddle and seat the vacumn effect of the boost blowing out causes the incoming fuel to be sucked out and the float bowl to go empty! This causing the engine to lean out and problems start to arise!

One major thing to remember when turbo charging is to make sure that at any given time you have more fuel pressure than boost pressure! (other wise boost is blowing the fuel back down the fuel lines!)
So if you are running 10 psi boost you need over this in fuel pressure say around 15 psi fuel pressure!
But with a carbie the needle and seat can't hold15psi fuel pressure unless it is getting used up straight away! So you need to run a malpassi rising rate ( carbie not efi ) fuel pressure regulator!

( One other thing is if your carbie has vacumn secondaries you can't boost it as the carbie won't work properly! )