I see notes here and there that I can mesh glinet routers but I dont see that lfunction isted in the router descriptions.
What routers can mesh, both wired backhaul and wifi meshed? Can I intermix routers? or do I need the same routers in all locations.
Mesh is not supported so far, I am sorry.
You can stack Wi-Fi networks, like using the extender or repeater mode - but this isn't a real mesh like you might know from Ubiquti, LANCOM, AVM or MikroTik
Is want to test these ones: GL-B1300 / Convexa-B - GL.iNet
Depending on your needs and your network experience, maybe this ones are more suitable: GL-B2200 / Velica - GL.iNet
The Products Page got a checkbox on the left for Mesh-Wi-Fi -> GL.iNet Routers and Gateways Selection - GL.iNet
The B2200 is nearly EOL - not technically and by support, but the OpenWrt version is 15.05 and there won't be any upgrade soon.
The B1300 could be fine indeed, those series are the only one I did not test yet. 
On one side I am very interested in the Velicita, on the other side I don't know if it is 'real' interoperable client AP switching as in 802.11r, not the 802.11s MESH for routing between the APs.
I got these both standards confused a long time, thanks to the marketing misuse of proprietary 'mesh' networks.
I would even say that both standards are not the real cool thing. Meshing makes only fun if the APs are able to connect via 5 GHz and 2.4 GHz the same time.
Just get a brume 2 and use it as your main router then buy ANY mesh network like eero or netgear or linksys and use them in bridge mode until GLiNet releases a proper mesh router.