Adam Funk wrote:

Are the cans & guns standardized & interchangeable

Yes.

Do the foam cans seal themselves well (like gas cylinders) when taken
off the gun?

Err, usually!

http://andyburns.uk/test/foam_leak.jpg

Do you leave a can of cleaner on the gun when it's not
in use, or just blow cleaner through the gun & then leave it open?

If I'm not leaving a can of foam on it, then I tend to leave some
cleaner in it instead, it's difficult to say how long is "too long" to
leave a part used can of foam on it, days is fine, weeks is fine, months
usually causes [me] problems.