I think testimonials aren't worth the e-paper they're written on. I could write a dozen testimonials right now on any subject you care for.
Similarly, a large sized team might just mean someone has been raiding Facebook profile pix. On the other hand, how are 17 people making money from a product they have been giving away for a year or more?
I have nothing against Tawk.to at all, but I think your broader advice of evaluating a company based on apparent user testimonials and the size of their apparent development team is nonsense.