My playtest results...

[eleran]

It depends on whether you interpret the "immediate" in "immediate melee basic attack" to mean "at once" or "an immediate action". If the latter, you get only one immediate action per round.

Excellent point! I interpreted it like an OA rather than an immediate action. Thanks for shaking me awake.

So, you could effectively swarm the fighter with minions if they do it all at once. The old "mob attacks because you can only kill one of us" routine.

 

[HP Dreadnought]

I'd think it would have to be an immediate action. That is the only way (besides free) to take an action on somebody else's turn.