The G54 we will have to treat as a third party router because you flashed it with 3rd party firmware. It is ok that you mentioned that you have it, but just as a warning, do not say anything else about it.
As long as you planned on wiring these together then you shouldn't have an issue. If you are trying to connect the routers wirelessly it wont work.
To bridge the G300NH, just put the switch into bridge mode and hook it up to the other router via the LAN ports. I would also suggest that you log into the bridge to give it an SSID if you want to connect to it specifically. The bridge IP is 192.168.11.100