I have seen a lot of comments about fish and they way they will or won;t get along. Skunk Botia loaches not getting a long with anything with long fins. Rosey Barbs taking out anything else they are put in with. Angels fighting with anything smaller than they are. Bettas killing each other regardless and so on.
While I don't have a lot of experience with mixing fish I do have Skunk Botia Loaches and they have never injured another fish, ever. I have kept them with Swordtails, platies, neon tetras, cories, BN plecos, and even with Bamboo shrimp and they never hurt anything.
Another one was a friend of mines tank. He had a 125 gallon tank and had 1 Albino Oscar, 2 Jack Dempseys, 1 Red Devil and a common pleco. These fish never touched each other unless it was by accident when they tried to eat the same food. They were raised from very small in the large tank and never fought.
When looking into raising fish, especially those that can be aggressive or predatory find out what type of fish they had been kept with if you can. Fish will adapt to each other and it would surprise a lot of people what type of fish can be kept in the same tank if done right.
Is kinda like when I raised Gerbils. If I put the babies together when very young they never had a problem but put two adults together that hadn't been in the same cage and one was going to die if not both because they would fight to the death.
There are those exceptions and the main one is the Betta. Mid juvenile male fish let alone adult males will not get along with each other but a single male raised with females from very young will get along with them. In the wild the males are surrounded by possibly 100's of females in the same rice paddy or pond. I would guess there are many other species that males or even females will never get along so always research them to find out. And I could be wrong about male bettas but I have never found anything that says they will not fight and try to kill each other once they reach adult or breeding age.