Most tetras get along very well with tropical community fish such as guppies, platies, and mollies. Depending on the size of the tetra, you may also be able to put some of the more docile semi-agressives in there as well. To be safe, avoid putting anything in your tank that can fit your tetras in it's mouth.
Yes
There are millions of tropical fish in the tropical oceans. But they are all marine fish not freshwater fish. Freshwater tropicals can not live in saltwater.
I wouldn't risk it. betas live alone, and most betas will kill company. buy a separate fish bowl, appropriate for the tadpole when it is older, and larger. :)
Betas are brown in the wild because some Betas live in mud, and feed off of flys and small insect's. Betas are livebearers. Betas also somtimes eat other fish.
Neons
No. Tropical Fish required warmer water.
Most tropical fish live in the ocean, but some do live in freshwater.
No! because tropical fish live in heated water and koi in cold water.
None. Their temperature requirements are way different. Betas are tropical fish, preferring water between 82 and 86, while goldfish need water between 72 and 78.
Tropical fish.
No, it will not die, but don't be worried by your beta attacking a goldfish. Betas are usually only violent toward other betas, so your goldfish is safe. Also, I believe betas are tropical fish and goldfish are coldwater, but I have heard many sucess stories of betas surviving years in coldwater tanks and bowls. Best of luck
parrot fish live in tropical waters