Are any of these species incompatible with each other?

-Platys
-Mollys
-Swordtails
-Corys
-Plecos
-Killifish
-Tetras
-Neon Tetras
-Loaches
-Guppies
-Hatchets
-Danios
-Discuses
-Gouramis
-Bettas

Just curious. Thanks.

Discus are cichlids and will eat smaller fish if they can catch them.

Ditto on the discus.Discus should really only be kept with discus or maybe a few bottom dwellers.

You really cant just go by gouramis, tetras or things like plecos because there are a WIDE variety, some being very peaceful(example:rummy nose tetras) and some being completely homicidal(example: emperor tetras).

Here is a list which will live together the best with no little problem:

-Platys
-Mollys
-Swordtail
-Cory
-Guppies

Here are some fish that will be ok with the list above but the water condition and requirement is abit different, also be sure that you have enough space because they are schooling fish, so get atleast 5 fish for each:

-Tetras
-Neon Tetras

Betta is best keep alone, I know a few people that keep betta with guppies, but they have enough space, a big enough tank and plenty of hiding space, because sometime will attack them. I wouldn't risk it.

I wouldn't keep gouramis and betta together, wouldn't risk it.

You can keep plecos with your guppies and the rest, but be sure there is enough space, because there are certain type of plecos that can grow up more than 1 foot. So they can require up to 100 gallon. Like Sailfin Pleco, they can grow very big.

You could try a female betta, but I had a male betta bite the tails completely off some guppies before
Also, my betta and gourami would have 'show downs'...never really fight, but I think they really stressed one another out.

I agree, bettas probably ought to be kept alone. I have danios and neon tetras together, and they seem fine. I don't think danios should be kept with guppies, because they might nip at the guppies' tails.

I've also heard that gouramis can be aggressive.... though I've never had any.

Here's a general resource on compatible fish that I've found very helpful: Fish Compatibility Chart

Some of my experiences--

I had a redeye tetra and serpae tetra that attacked my cories. Also a balloon belly molly that ripped up one of my cories when it was sick.

Swordtail males are territorial so only one per tank. I did have a swordtail male and a male platy get into a fight once--no harm was done but they really went at each other for a minute or two. I had to separate them.

My male betta went after my male and female swordtails.

Out of curiosity, what size tank are you looking at? I bet you could get all those fish to get along together with a nice 250 gallon tank with a lot of decor arranged correctly. Not all of them like the same pH, temp, water movement, salinity, etc., though.