USB-3 Type A Connector Current Drain
USB 3 Type A (i.e. 9pin connector) port must provide AT LEAST 900 mA if the host system is powered from AC or with a sizeable battery. So draining 900 mA should be fine.
However, what if your device gets plugged into a regular USB-A 4-pin port? These ports are mandated to supply AT LEAST 500 mA. Is this sufficent for your needs? Of course, 95% of USB2 ports would provide quite a bit more than 500 mA, but there are some (rare) cases when USB ports have overcurrent protection in hardware tuned to 600-700 mA cut-off.
Therefore, to comply with USB standard, you should include some detector of whether the port is USB2 or USB3.
One way to detect the presense of USB3 port is to have 45 Ohms termination on Rx+ and Rx-pins, and check if there is electrical activity. The USB3 host, when it's Tx sees termination on the opposite side, will periodically issue Rx_detect pulse, and then send LFPS signals (10 kHz bursts with 500-600 mV amplitude (1V p-p differential), for 360 ms. This activity should be relatively easy to detect with any MCU, and make an appropiate decision to engage or not to engage.