I'd say try it and see how it works. And the things about swordtails, they didn't last a month in my tank. They disappeared one by one. They probably ate it when the lights where out.
10, 20 makes no diff. like I said in the 75 gal my Female con harrasses other fish and these are bigger fish. If they were neons she would kill hem for sure.
My other two cons are in a 10 gal and the male there is barley tolrant of his mate in that tank.
remember for their size cons are probably one of the most territorial and aggressive fish.
But if your determined to give it a try go for it. all you will be out is a few feeder fish. what 10-20 bucks if you get um on sale. And you will have a fat Con when your done.
Whatever. I still stand that a single female con would do fine in a community tank with neons. If it is just a female she won't get that aggressive because she doesn't need to guard fry or eggs.
Females can and will lay eggs, without presence of a male, and protect them until she has finished eating them or until some other fish eats them, and then proceed to do it again, just as she would if a male was present. You're still getting eggs and the protective behavior but not fry.
Breeding related aggression is less to worry about (IMO) than her getting bored and eating one out of hunger or boredom or instinct or whatever. When protecting, they chase to make a fish go away. When attempting to eat a smaller fish the con will seek it out until it is satisfied.
Female cons may stay considerably smaller than males but it doesn't mean they stay small enough to keep from fitting a neon tetra in her mouth, or at least pices of one.
Not all cons are killers, some are quite peaceful, some females don't lay and protect their infertile eggs on a regular basis but I'd rather not risk it.
You can get feeders for mere change, see what she does with a bunch of inexpensive 1" guppies (healthy ones) first.