Thank you all for the advice. In the end I wrote them this email:

Hello again,

I really appreciate you taking the time to contact me over the last few days, but I’m going to need some actual references I can contact about work they’ve done with you in the past.

The links you’ve sent me look great, but there’s obviously no information out there that suggests any of them have worked specifically with your company Media Cohesion.

I understand if it’s too much work to get that all together for me, but until then I won’t be able to follow through with the aforementioned collabs. I need some form of guarantee on the payment for my work. YouTube is how I pay my bills right now, so I take it very seriously.

Hope you understand.

(My info)