I'm afraid many of those fish listed here are highly incompatible.
I'm going to go off of your tank size here, which is 10 gallons.
Shrimp sometimes work, as will mystery snails. Sometimes docile bettas can be kept with platies . If you had a much larger tank (20 gal) you could also get a group of kuhli loaches, minimum of 3 though as they like their own. You could get the smallest cory catfish for a 10 gallon, as they also like the company of their species. I have heard of neon tetras working before as well.
Tetras will not work (with the exception of neons) as they are generally known for being nippy and a schooling fish.
Plecos get way too large for a 10 gallon, including size and bioload.
ADF are also not a good idea, based on what I have heard from others here - someone else will have to chime in on why.
Oto catfish like groups, and need a large amount of algae and are also sensitive to water conditions. They are not recommended for beginner aquaraists.
Danios are WAY too active and will stress out the betta.
Guppies have long fins and can look like competition to the betta.
Gouramis are from the same sub species as our beloved betta, and also will not work as they will fight.
Cichlids are too territorial.
I can't comment on swordtails honestly.
Mollies will not work as they are brackish water fish. Tetras will not work b/c they are too nippy.
You must research any possible fish you would like to keep, including filteration levels (bettas like slow moving water and with their heavy finnage can be blown around by a strong filter), heat requirements (betta's like their water 78-80), whether or not it's a schooling fish (space requirements) or a nippy fish (bettas fins will become fair game), and also very colorfish fish can be viewed as competition to the betta and he may fight them.
In all honesty, you are best off with bottom dwellers as tank mates, and when in doubt leave betta by himself.
1. that betta water is expensive... so u should by dechlorinater stuff, they have it al walmart.
2.they will eat the ghost shrimo, its a good snack but not a good pal...
african dwarf frogs
i hope that helped :D
and condolences for your long been deseased goldfish :o
EDIT: and with sororities you have to have 4 or more females not 3 or less...they bully eachother
can we see pictures of your betta Triton?!?! ;o
EDIT AGAIN: you can also get dividers for your ten gallon if you want more males, or you can make dividers, there is a thread on here somewhere on how to make them :)