!rant => question?

I'm hired as a freelancer for a start-up that wants to create a social-network-like platform. I've always been a "basic" PHP and javascript developer like using AngularJS, MySQL and my own kinda like PHP MVC framework etc.. But I'm worried that 'this' will come short when the platform expands on the user-base and stuff. That MySQL won't be able to keep up with the expectations and the amountof data, that AngularJS will not be enough for the Frontend,.. I've taken a look at ReactJS, RethinkDB, NodeJS and such, but this is not really within my "comfort zone" and I'm not willing to invest time in something new if it's not able to handle the platform (I don't know if it will..) and I'm afraid that I'll have to start from scratch if it all fails.. (and this is something I can't afford)

So.. What are you guys's opinions? We're not looking at millions of users, but it will have feeds, comments, connections, messages, post scopes,.. Etc. RethinkDB looks promising with the 'watchers' to get live data instantly, but it's a whole new way of query'ing and such.. It just feels like I'm wasting my time because I'm afraid that I'll reach a point in development where I'll have a situation for example like "damn.. This is impossible with angular or php.." [I've shouldn't have agreed to this project..] :D