Ideally it would be based on general politics and economics. The Frost Giants would generally look down on the dwarves and the dwarves would find some of the Giant's habits distasteful. But they may get together to trade occasionally, since the Giants would want dwarven metals and gems and the dwarves would want some of the tundra items (like Winter Wine) or animals. War Mammoths, anyone? But most of the time the Giants would ignore the dwarves or raid them for their goods instead of trade. Dwarves in hot climates should never see a Frost Giant (although there will be other types around eventually).
The Hobgoblins might trade occasionally, but should generally prefer to simply conquer the dwarves. Depending on the political landscape I could see the two even having an uneasy alliance, though, since Hobgoblins at least can be trusted more than the other evil races.
The Dyansauri are savage and brutal. They might trade with the dwarves if they're busy fighting some of the other races, but would generally prefer to raid for what they need. In this case I think the dwarves would be the ones refusing trade, since the Dyansauri aren't too trustworthy and have a habit of eating people.
For now, though, I may just stick them in a folder and give them two entity files for you to pick from. One that's neutral and one that trades. It's a bit more maintenance work in the long term, but the flexibility might be worth it.