I have a Mac only, so I am not able to get Terraria through Steam. I was wondering if I can play Terraria on my iPad Mini with a friend who has it on his PC. I was wondering this, because I saw that Deepworld on iOS is compatible with the Mac version.
The mobile and PC versions of Terraria have different content and are not compatible with each other. This might change in the future, but it seems unlikely at the time.
As for you mac,
I have three friends with macs, and over 50 friends with pc's who play Terraria. However, my three friends and I still manage to get together and play Terraria on my PC server using V-mare fusion. Using it, they launch windows on their mac, run steam on it, and then run terraria on steam. Its a great solution and as far I'm concerned it should work for you.
The best part is, that V-Mare might take some time to boot, but once its up you can slide between windows and your mac. Its simple!