1) Not at all. They are similar only in that they are both board games played with miniatures and dice.
2) The cards affect play in different ways in Calth and Prospero. In Prospero they are used only for the psychic phase. The T-Sons get psychic power cards and both sides get casting or dispel cards. The T-sons flip the casting cards to try to get a spell off, the Space puppies and friends get to flip dispel cards to try to stop them.
In Calth the cards are mostly for special abilities you can use in combat, but there are dreadnought cards which show where you hit the dreadnought when shooting at it.
3.) Yes. The whole T-Sons force is made up of Librarians.
4.) That is tough to answer. I don't know that there is an upper size limit for Calth in terms of it bogging down. I'd guess that 3x Prospero sets would be the limit there. Of course a lot depends on your tolerance.
Let me state that Calth and Prospero are different games. More similar to each other than either is to Space Hulk but not inherently compatible at all.
I prefer the scenarios in Calth but like the mechanisms in Prospero better.
Over all neither one is as good as Space Hulk and most people are buying them for the figures. I think Calth has more replay ability than Prospero but neither is as engaging as Space Hulk.