
The theory is that by exposing him to places where other dogs have been (i.e. the ground), you risk him picking up something nasty that he isn't vaccinated for yet, like parvo. What you can do - this is all discussed in another thread, btw - is look into socialisation classes at your vet - I took both my pups to them, they are a very good idea. Plus, the other pups will be at the same stage of vaccination, so the risks are minimalised.
It is controversial though - many people will say "don't put them on the ground, but they are fine round dogs you know indoors" - which presumably defeats the purpose, as those dogs will have been outside and possibly been exposed. Oh, he won't catch anything off the cat!
The bordatella cavvine is a single shot - and not a nice one, either! The vet gets a little syringe full of pinky-purply vaccine and squirts it up the dog's nose. most dogs hate it, mine certainly did - but if you will be kennelling him, it's a good idea.
Sorry, I don't know about spacing or if they can all be done at once - I would imagine it would be better to space them out, to give his immune system a chance to develop antibodies to each vaccine in turn, without overloading it. Do whatever you're comfortable with, same goes for taking him outside.