Or does it have to be all of it, whether for that particular currency or all the currencies held?
For example, if I have 100 usd worth of each X, Y and Z coin, can I withdraw only 10 usd worth of Y coin?
Yes, but there are two limits that make you think before you withdraw, one is the withdrawal fee, which varies by coins, and the second is the daily limit for withdrawal, which is 0.06BTC for all accounts (Some accounts have 0.05 limit.)
you can track fees here --->
https://withdrawalfees.com/coinsYou can reduce the fees by withdrawing bitcoins using BSC and then trying to exchange them on any platform with lower fees instead of 5 dollars0.005BTC per each.