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?
You should be able to but you'll never know when they'll strike you with a KYC mandate. It happened to me before. I was trading and withdrawing happily and one day out of the fucking blue they decided to lock my acc and ask for a KYC. Since the amount I was holding that time was quite big, I had to comply. Everything is going fine since then but I feel like I was tricked into this. Sooner or later they'll ask for a KYC. It'll happen to every single active unverified account there is.