Peaceful in every way that usually matters
On the questions community keepers usually ask, Endler's livebearers pass easily. They do not nip fins, they do not chase or bully other species, they hold no territory, and they use the upper and middle water in a busy, good-natured way that leaves bottom dwellers and shy species alone. Their small mouths and size mean adult tankmates of similar or larger size are in no danger from them at all.
Their activity can even be useful in a community. A moving group of confident little fish signals safety to shyer species, and many keepers find that nervous tankmates come out more once Endlers are cruising openly above them. Few fish this small bring that much life to the top of a tank.
Caveat one: the males never clock off
The one behaviour that does cause friction is internal. Male Endlers court females almost constantly, and in a pair or a male-heavy group a female can be followed to exhaustion. The fix is stocking, not separation: keep several females to each male so attention spreads, and give the group plants to break lines of sight. Males spar briefly with each other in harmless display contests, but they do not injure rivals.
That same drive makes them poor companions for long-finned, slow fish whose flowing fins attract persistent investigation, and for any fish stressed by constant motion around it. Peaceful does not automatically mean relaxing for every species.
Caveat two: babies, and lots of them
Endlers are livebearers that breed with enthusiasm, and a mixed group in a comfortable tank produces fry regularly. In a community, most fry are eaten quickly, which keeps numbers stable, but in a planted tank enough survive to crowd a small setup within months. Plan the endpoint before you start: a males-only group for colour without fry, a species tank where you raise young deliberately, or a community with enough tankmates that numbers hold.
Relatedly, Endlers cross readily with guppies, their close relatives, producing hybrid young. Keepers who value either strain pure should not house them together, however well they otherwise get along. The breeding habit also changes how you judge a fair stocking level: a tank that looks comfortably empty with a fresh mixed group can be fully stocked within a few months on fry alone, so count the fish you will have, not just the fish in the shop bag, and have a plan for surplus males, whether that is a second tank, a friend's setup, or returning them to a willing shop.
Caveat three: they need protecting themselves
Peaceful fish are often the victims rather than the problem. Nippy species will shred the males' fins, larger fish will eat Endlers outright, and fast, greedy feeders can leave these small fish underfed at the surface scrum. Choose companions that are calm, similarly sized, and not fin-nippers, feed in more than one spot so the Endlers get their share, and their community life runs smoothly for years.
Local experience is gold when you are matching tankmates, because what works depends on your water and stocking, and KinsLe's Circle of Love connects free accounts into a wider local network, so nearby pet lovers can help with advice when you are planning an Endler community.