Depends on the size of tank. Danios are very active little things and really need a 3 foot tank to get an outlet for their activity needs. They also need to be in a sizeable school, at least 6-8. Smaller groups of danios, particularly if confined in small tank, are likely to develop aggression issues.
As long as there is enough size, there's no reason they shouldn't get along with the common livebearers.
Guppies and platies are fine in a 2 foot tank. With them, it's more a question of gender distribution.
If you keep just a couple of males together, or males with not enough females, you may get aggression problems. A large group of male guppies (say at least 6) usually works well- as agression is spread. Females on their own works even better, but most people don't like them so well because they're less pretty. But very nice fish. If you want mixed sexes, a ratio of 1 male/2-3 females is fine- but they will breed like rabbits, so you'll need to rehome the fry. Also note that females can store sperm and may already be pregnant from the shop.
More or less the same goes for platies, though I have found platy males very temperamental.
In my experience guppies and platies get on very well together.
Mollies are more difficult to advise on as they are several different species- and hybrids of several different species. They can get very big, so need a sizeable tank. Also, they tend to be more sensitive to water stats, a lot of them really do need hard alkaline water, and many even prefer brackish. I tried to keep black mollies in a soft water tank when I was a child and they were constantly falling ill. The same thing about gender applies to mollies as to guppies.
Other fish that go well with the livebearers are bottom dwellers such as corydoras, khuulie loaches, bristlenose plecs (if tank is large enough). Note that corydoras and khuulies are schooling fish, so you need to keep a group of at least 3-4.
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