Is it OK to Associate With Non-Buddhists?

In the orthodox traditions of several religions, associating with non-believers is either discouraged or forbidden. People clearly have a certain tendency to associate more closely with members of their own "group" and find ways to justify it using scripture or philosophy. A number of religions make this easy.

The question is, is it possible to come up with some sort of justification like this using Buddhist doctrine? Could the most orthodox Buddhists come up with something?

In the US where I live, only 0.7% of the population identifies themselves as Buddhist, and the US tends to be a rather culturally heterogeneous environment anyway, so not surprisingly you don't really see Buddhists in the US refusing to associate with non-Buddhists; for the most part they can't refuse even if they wanted to. But in some countries where more than half of the population is Buddhist, it would be quite possible to only associate with other Buddhists most of the time.

But is there anything that would actually tend to encourage or justify this kind of behavior in the Tipitaka?

The first thing that comes to mind are these famous verses from the Dhammapada, 328-330:

If you gain a mature companion —
a fellow traveler, right-living, enlightened —
overcoming all dangers
go with him, gratified,
mindful.

If you don't gain a mature companion —
a fellow traveler, right-living, enlightened —
go alone
like a king renouncing his kingdom,
like the elephant in the Matanga wilds,
his herd.

Going alone is better,
there's no companionship with a fool.
Go alone,
doing no evil, at peace,
like the elephant in the Matanga wilds.

There are a number of places where "companionship with fools" is discouraged. One might try to interpret "fool" to mean anyone who isn't Buddhist, but the definition of "fool" is mentioned in several places and is not defined in terms of how one identifies oneself:

"Monks, these two are fools. Which two? The one who doesn't see his transgression as a transgression, and the one who doesn't rightfully pardon another who has confessed his transgression. These two are fools." -AN 2.21

"A person endowed with three things is to be recognized as a fool. Which three? Bodily misconduct, verbal misconduct, mental misconduct. A person endowed with these three things is to be recognized as a fool." -AN 3.2

"And how is a monk skilled in characteristics? There is the case where a monk discerns, as it actually is, that a fool is characterized by his actions, a wise person is characterized by his actions. This is how a monk is skilled in characteristics." -MN 33

"It is through living together that a person's virtue may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.

"It is through dealing with a person that his purity may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.

"It is through adversity that a person's endurance may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning.

"It is through discussion that a person's discernment may be known, and then only after a long period, not a short period; by one who is attentive, not by one who is inattentive; by one who is discerning, not by one who is not discerning." Ud 6.2

And, to provide a quick summary for a layperson's code of virtue:

"What are the four vices in conduct that he has eradicated? The destruction of life, householder, is a vice and so are stealing, sexual misconduct, and lying. These are the four vices that he has eradicated." -DN 31

Not to mention "indulging in intoxicants" due to their tendency to cause misconduct. And of course there are many other places where these things are defined similarly; I've just picked the few that are easiest to find online.

So, while Buddhism discourages "associating with fools," many (hopefully most) non-Buddhists would not be considered fools by the Buddhist definition. People are defined by their actions, not by how they identify themselves.