Hey ChrispTho, how's it goin?
In regards to your Gourami, all I can give you is my experience with them.
I currently have two male Gourami's in a 15gal tank amongst other fish (6 Raspboras, 1 Red Wag Platy, 1 Snail & 1 Bamboo Shrimp)
They have both been in this tank for a little over a year now.
When I first got the 1 Gourami there were Raspboras, Platys, Snails and a Bamboo Shrimp within the tank.
These fish were added both before and after the Gourami was introduced.
So far, He has had no issue adapting to other inhabitants.
However, to answer your question.. The second Gourami was not introduced into the tank until 4 months later (after the Raspboras, and Platys) and I have had no issues with the "Older Gourami"
They chased each other here and there but have had no loss of life because of it.
I've never had any "blood shed" in the tank because they were territorial and are both are growing well.
Perhaps when you add the neons, take out the Gourami "temporarily in a cup/net combo" move the decor around and then place him back in.
In Theory, perhaps this will confuse the fish thinking its within a new environment and therefore it will think that the neons were already within the tank.
I (based on experience of adding and removing "new" fish to an existing Gourami tank) cannot forsee any issues.
Let us know how you make out!
Happy Fish Keeping