Your questions about SoFi Hong Kong are answered here

The monthly platform fee will apply if you have at least HK$30 worth of net assets (Cash + Stock) with us. The monthly platform fee will be deducted from your brokerage account.

If there is insufficient Hong Kong dollars, your account will be in negative balance and may start to accrue interest. If you have any other currencies in your brokerage account, we will automatically convert them to cover your negative balance on the next business day.