I'm trying to understand how some of the math with margins goes when doing cross currency trades. I haven't found anything that goes into it, so I'm kind of at a loss. Assuming I start in USD, and I want to trade some X:Y pair, how are the lot sizes and margin rules handled? How should I assert stop losses for such orders? Beyond that, I am curious about how the trades execute, given that some of these pairs aren't as widely traded.
Have you asked your broker? Here is how IB handles FX margin determination ... https://www.interactivebrokers.com/en/index.php?f=margin&p=forex