Communication /collaboration app

I am about to start a really big and complicated project. I also just signed up with Zoho-One (as Solo-preneur). I hope to be able to use Zoho to organize and coordinate this project.

I recommended Zoho to one of my collaborators. Which app(s) should we use to communicate? Cliq, or another app? Or shall we stick to Slack, after all?

Also, it seems that it is not possible to add collaborators to Zoho-Projects if you have a single account. How can I still collaborate with others?