Trying to think about this, but is it possible for some of the freshwater diseases to spread to a saltwater tank by either adding fish that can be mixed in (Mollies for example) or by sharing equipment?
No, there are some that are similar but they cannot live in either fully freshwater and saltwater. If you have a fish such as a Molly and it gets something, it cannot spread to a saltwater tank since the underlying species of bacteria/virus is different.
When it comes to parasites, they can infect others from saltwater or freshwater.
I know there are some that are the same, but I think they are different subspecies (can be in the same realm). Which is why using salt for some diseases and definitely will kill any parasites.
Maybe some viruses could pass but seems unlikely.