USB3 SSD on Rpi

I am thinking of buying a portable SSD to use for storage on a RaspBerry Pi.
It seems that USB2 devices have been replaced by USB3. I know they are supposed to be backwards compatible but I am concerned with the current drain.
If you plug a USB3 device into a Rasberry Pi USB2 slot will it drop back to USB2 current spec?

Yes, it will drop back to USB2.

I have a Crucial BX300 SSD on a 2nd generation RPi 2 and the current drain is almost negligible (although, so is the data throughput.)

I use it as the default drive (SCSI::4) and it works perfectly. (the SD card is only there to hold the boot files)