I have a 3.5" HDD enclosure with external power supply. Inside I have a 2.5" HDD and there is space in there for the RPi. The problem is I don't want to have two power supplies so I was thinking to solder a connector from the 5V S-ATA power connector to the USB RPi power connector, both being 5V (parallel connection).

I'm no electronics technician but would the internal resistance of the devices interfere with one another?

I will also solder the usb hdd link to the one of the RPi's usb host port if this setup works.