Some good advice there from Nici - I'd just add to take the pup outside for toiletting as soon as he's eaten and as soon as he wakes up. As Nici said, the important thing is to praise when the pup toilets outside, and not to tell him off if he toilets inside. If you tell him off for toiletting inside he won't make the connection that it's because he's done it inside - he'll just think he's being told off for toiletting and then he'll be reluctant to go anywhere he can see you - either inside or outside.

The way I introduce a new pup to the cats is to use a crate - put the pup in the crate and let the cats come into the room so they can suss him out. Once you are satisfied that they are not going to freak, you can let them interact for short, supervised periods, gradually lengthening this time over a period of days until you are satisfied that they are going to be fine.