If i buy a Flint 2, can i buy a Velica and use it as a mesh for the wifi?

Im not new to networking, but i have never dabbled in mesh networking, since i havent needed it until now.

Can i buy a OpenWRT router like the Flint 2 and then add their Velica wireless routers as a mesh? I basically dont find any information about adding mesh routers to an already existing router setup. I have the need for both wired and wireless.

No, you can't.

Mesh is officially only supported between Velica routers.