I don't know for sure, but in general probably not no. Dolphins tend to be pretty aggressive towards other marine life. However animals tend to be far more complex than they're accredited for, especially dolphins known for their intelligence, so yes, almost certainly at some point in time dolphins have had playful interactions with other creatures. The world is so so so much greater than we ever appreciate. Animals find friends in the most unusual places all the time. THey do funny and unpredictable things. There'll have almost certainly been an incident of a dolphin becoming best friends with a seal for life, it's just at which point, over the last few million years was it? there's been a hell of a lot of dolphins since then :)

Well, they're well-known for ganging up on porpoises and killing them.

I know that they sometimes play with certain types of whales (blue whales and humpbacks come to mind), but do they ever playfully interact with seals, sea lions, other species of dolphins, belugas, etc?