LOL, I have 4 different instances of NodeRed running on 3 different Pis and 1 beefy homeserver. 
NodeRed is actually fairly lightweight in and of itself. The more you add, the heavier it becomes. One of my Pis has a NR instance, MQTT (Mosquito) instance and a MariaDB server on it and runs like a champ.
As for NodeRed to HE, I use the Maker API as a simple means of communication between the two. When NodeRed needs to flip a switch or turn on/off a bulb, I just shoot a HTTP GET request to the Maker API. For HE to NR, I use websockets OR a HTTP GET from HE to NR. Easy peasy.
You could go the MQTT route as well, however, that part is still in Alpha on HE. But, half a dozen of one. 
Also, to give you an idea of what I'm using NodeRed for, check out my thread on it (although, @stephack has pretty much taken over as the master of knowledge there lol):
