Welcome!
That's a lovely little tank, I can see you've put a lot of thought and effort into the scape, and it looks so much better than most first time tanks, believe me! Wish my first one looked that good when I first set it up.
I found all of the conflicting information out there overwhelming at first too, and still do at times. There are some places online that people will really shout out and humiliate newbies to the hobby for any mistakes they made or info they didn't know, which doesn't help matters. The best way I found to wade through it all is to look for the reasoning behind the rules you hear repeated. Like why bettas should live alone (which is still up for debate for some people, who have had success with them in community tanks). It's not easy to find real research, and a lot of myths persist about bettas, but you have to dig for real sources. Your reading material makes me believe you'll have no problem doing that
So for example, in the wild, bettas live in large but relatively shallow waters, and we've designed the domestic varieties to have large colourful fins which create a lot of drag, and make it harder for the fish to swim easily. They also a labyrinth organ to breathe from the surface, as well as taking air from the water. So when people talk about having a tank that has a good amount of swimming space horizontally for them, but not too deep of a tank, and to use some large leaved plants close to the surface for them to rest on- that makes sense. They like to rest and sleep on leaves near the surface where they can easily go up to breathe, and they can't cope with a heavy flow on a filter because of those big fins, and they evolved to live in very still waters without a lot of flow to battle against.
They also rarely encounter other fish in the wild, since not many other species can cope in those water conditions. Males stay in one territory, while the females travel looking for males. They'll court, or fight, but if they court successfully, they'll place the eggs in the bubble nest and the female goes on her merry way, leaving the male to guard the nest and the fry. This makes them fiercely territorial fish, and although some have had success keeping them with other species, it usually means the right mix of fish in a big enough tank for them to get away, and fish that won't nip the bettas fins. And we don't know whether having other fish in "his" territory will cause a low grade, chronic stress to the betta, so is it worth forcing it? A single betta could work in your tank, but not with neons too.
Neons like in massive groups in the wild, and they're a slender, delicate, darting fish. Seeing a school of hundreds swimming in a huge tank is a show stopper. Check out this video. It's cardinal tetra here, but they're so similar, they'll even school together, so it's the same as neons. Check out the schooling and how fast they move when they eat[
warning, obnoxious loud music, might want to mute or lower volume first]:
So personally, I wouldn't want to keep neons in a small group in a small tank, where they can't really express this behaviour without hitting into the side of the tank. I don't know whether a single gourami might work, but it's definitely too small for two of them - they can be a territorial fish too, and do some real damage.
Consider some nano species. Nano tanks have really taken off in recent years, and some new, very small but beautiful species have been discovered that can work beautifully in a nano tank, like celestial pearl danios. Gorgeous little fish that would thrive in a small group in that tank. Check those out and tell me you wouldn't want some
There's a section in the forum for nano tanks, which yours is, why not search for some nano tank inspiration there and elsewhere, make a list of things and ideas you like, then research, research, research. If you'd still like to have neons and maybe gourami and other types of fish together, you'll really need to get a bigger tank. Bigger tanks are also easier to maintain than smaller ones, believe it or not, since the water conditions remain much more stable and things can go wrong quickly in a nano tank with not much water to dilute ammonia/nitrites/nitrates.
Sorry for the essay, but I hope some of it was helpful and didn't come across as critical
