Killies are not your average tropical fish. First, they JUMP! They will try to escape out of your tank in a way no other fish will. ANY tiny opening and that is it baby, they are out of there. They're also not the, umm, brightest fish around. I have had many swim in to on under filters, get stuck and die. You need to "child-proof" the tank.
Killies can be mean. The supposedly delicate Aphyosemion ausrale can at times just bite the eye out of a rasbora for no apparant reason. Naturally they'll do this in your kids tank, the one you're trying to see if you can get away keeping a killi or two in. They're best kept alone. Sometimes. Other times they're ok. Other times they get beat up.
Their temprement is uneven in most cases. I've seen them be mean and cowards - different fish of the same species. Australe are supposed to be timid, not in my experience. Gularis are supposed to be aggressive. They're not, they seem to be confident but never agresive. Males are more aggressive that females, as in most fish, but, I've seen pairs where the female just kills the male and right quickly. To play it safe with a single pair I separate the two. That way I know they're not going to kill eash other. With a bunch of fish you recieve as a colony, say 6 fish or more there's a good chance they'll get along. I like to throw that many Aphysemions into a 20 and pick out fry from the hornwort.
Any fish sold in a petcshop is more or less ok with any fish it can't eat. These ones are usually not bullies; they're not good petshop candidates.
In reality they'll eat anything. I can't say I've ever seen a killifish that wouldn't eat flake food - in the worst of cases - I recall one pair of Cynolebias - introducing dither fish helps the more neurotic fish to figure out those floaty bits might be edible.
They really don't care that much about temperature. A few species don't like it warm (Diapterons) and nerly without exception all can tolerate cold very well. Heaters are generally not seen around killifish in all but an arctic environment. Having said that, a few South American killie are happy to swim in water with ice on the surface. Well, happy might be the wrong word but they don't die, that's the point.
As long as the water is clean and ammonia free they don't have exacting water requirements. Having said that killies seem to be the last to die in a tank truly augering into a green and brown slimey mess, and many activly breeding killifish setups are far from clean.
Fishes of the genus Nothobranchius are VERY prone to "velvet" and seem to benefit from sale salt in the water. Other than this, killifish are they're pretty disease free.
A couple of few of some well selected killies can spruce up most community tanks, but of course since the females are always drab and look a bit like female guppies you could not mix species whose females do not differ in shape if you hope they'll breed in this setup. In a tank with South American dwarf cichlids - Apistogramma and the like, surface dwelling killies (Epiplatys) of mid top upper swimming Lampeyes would be a superb choice and can be expected to breed freely among floating plants if well fed.
Non surface dwelling killies, Aphyosemion, Nothobranchius and related fish, generally do well enough with fish they can't eat or with anything but the worst bullies. Treat them as a platy or sword in most cases. A few species (A. gulare) do not tolerare their own species.
And then there's the South Americans. In general the non-annual fish, Rivulus can be treated the same. But, the annuals can tend to be a bit more delicate in some cases, both in terms of needing clean water and how easily they're picked on. At the other end of the scale, the giant Megalebias species can be treated as bass. Except they eat more. A lot more. These are not common fish and must be fed every few hours.
Serious killifans end up having lots of tiny tanks, with a pair in each, or just one of each sex. Medim sized tanks, 10-30 gals are used for raising batches of fry. While the temptation is great to breed great numbers of the newest and rarest fish to make a whack of money, the right thing to do is to raise the best dozen fish from a spawning of the best pair or colony you could find. You're a very serious killifan when you can be depended on to maintain a species or more, forever. This is more common in Europe than in America, but only slightly. You can always count on Al Castro for pupfish, Tony Terceira for bivs and Royal Ingersoll for Kethetys. Some things just don't change, this was true when I was a kid an is true now that I have kids of my own. Dr. Walter Foersh kept Nothobranchius foerschi for 40+ years.