Must remember, your bristlenose pleco is your algae eater/bottom feeder. If you add another then the one already there will have competition for food and territory. Bristlenose plecos eat a good amount of algae and in that size tank, I wouldn't be adding another algae eater/bottom feeder.
The reason you will want a larger group of neons would be because they are a heavy schooling fish. If their group is too small they tend to die rather quickly. The larger the group the better the survival rate.
If you're thinking along the lines of cardinal tetras then watch your water hardness first. Cardinals are not as accepting of harder water (pH, KH, Gh) as what neons are. I don't typically suggest cardinals to a beginner.
There are a wealth of options out there, please don't limit yourself so quickly. If you are looking for color there are male live bearers, and if stick to just males, such as guppies, you get all the color and none of the breeding. There are also other smaller fish that would do well with or without the skirts in that size tank, but are sturdier and easier to keep than neons.
galaxy rasboras *otherwise known as celestial pearl danio
danio species (zebra, leopard, long fin blues)
green fire tetras *one of my favorites
and also small species of rainbows that are options, such as
and celebese if you keep the skirts would also be an option, but I wouldn't mix celebese with neons or small peaceful, vulnerable fishes such as the galaxy rasboras..
If you really like loaches, maybe something like a small botia species.. lochata is a good one. While they do like to eat snails, they also do just fine on flake foods, frozen brine, and live black worms (not to be confused with blood worms) if u can find them.
There are alot of small colorful options for a tank that size. If you are looking for further interesting critters for the bottom feeding without adding another type of pleco/algae eater, you could try ghost shrimp, which are compatible with everything listed here... or ammano shrimp. Cherry shrimp would also be an option without celebese rainbows (adult celebese eat them sometimes).
There are many easy combinations I think would be much easier than neons, and surely easier than cardinals.
Please don't think you're out of options.
If you have questions about any of these species, please let me know and I can give you a basic run down on each of them.