Can they get along? Given the fierce and often fatal tribalism between homosapiens I'm inclined to say no. At best an earth with multiple human species would have caste societies. For example, neanderthals and the newly discovered "hobbits" would be relegated to lower castes like slaves or pets.
Have you ever scene an episode of any of the many Star Trek series? If not you will find your answer there.
Let's use the higher non-human primates as a stand-in for these hypothetical non-human hominids. Chimps, gorillas, orangutans, and gibbons are all heading for extinction in a matter of decades. There's your answer.
Let's bring it further along. Let's say we get off-planet and discover a planet full of backwards peoples, but reside on a planet laden with mineral resources that we need to ensure our survival or ensure our dominion. Looks like slavery or eradication all over again.
At one time apes were one of the most successful families of mammals. In fact, one geological era has been called the "Age of Apes". But the great apes have been wiping each other out for millions of years now. So we are no different than our predecessors.
Or just MAYBE, if we're to the point of colonizing planets, we MAY just be to the point where we have functional robots that do our "slave" work. Just guessing.
Maybe if the other human species kisses up to us enough we just might get along. We do get along with dogs, after all.
Naw, human nature indicates we will eradicate them, enslave them, or give them enough sustinance to survive, only to have their existance secondary to ours. The functional robots will do the eradicating or to enforce the enslavement, and to transport the mineral resources back to the home planet.