Chromis - how many?

[Xtasia]

Seems kinda pointless to "quarantine" them with his existing fish however.. Will be a tight squeeze.

Why not set up a seperate QT in rubbermaid container (barebottom with water from the 75g) with Powerhead, Heater.

I'm sure your purple tang will appreciate not having to share the 75g space in the interim. You get your chromis, and they are Qted for your main system.

[Aquattro]

I've never seen my other fish react in any way to the introduction of chromis. They just kinda look at them, say, "oh, some little dumb fish" and move on.
If it was me, I'd take advantage of group shipping and add them.