First, as for the two gobies, they should be fine.
Until the dragon goby gets REALLY large (nearing two feet) he shouldn't bother the knight goby. I have no experience with ones over 14" so I don't know if they then start swallowing small fish but I doubt it.
As for salinity, yes they should also bot be able to handle that.
For GSP's, you never know. If the tank is large enough, you keep the puffer well fed, and you give the goby enough hiding spots then it may work.
I've had several puffers with dragons and never had a problem - but you hear all the time about the GSP's going after them. So it may just be a tossup.