We need enough Batman to establish Dick's dynamic and history, and that's plenty; the Bat can easily overshadow any and everyone else so less is definitely more.
As for supporting cast it really depends on what kind of Nightwing story you're telling right? His most stable and longstanding supporting characters, in a way, are the Titans and there's definitely a way to use them without stealing Nightwing's thunder.
For something more grounded? I'm partial to the Dixon era supporting cast but there's been a number of people who've passed through Dick's life who might work, like Midnighter.
I'm not overly partial, honestly, as long as Nightwing is the hero of his own story and it's true to his character.
The only thing I require is as much Alfred as the script can possibly include.