About mounting PCs: a muadra riding a crugar would be pretty weird, but then, for most races, a lot of racial intermingling is weird, too. Ardoth is one of the only places on Jorune where two races choosing to spend time with each other wouldn't raise eyebrows. For most toth, each to his own. People from larger cities, as well as Drenn, would have no issues with it. Thriddle would find it fascinating; they seem to have few qualms themselves about freely interacting with anyone (though many people do have prejudice against thriddle - they are freaky monsters with bulging eyes on stalks, after all). Crugar tend to be prideful, and the affection and trust implicit in letting someone else ride you would be astonishing to crugar from the homeland of Temauntro.
On the other hand, some races are less alarming. The corastin - who are often relegated to menial labor as simpletons - are often bearers of others, for instance. But they're easily twice the mass of any other sentient race, and only a large bronth standing erect on hind legs would come close to the height of a corastin.