Can I use a Sora rear derailleur with my 7 speed shifter?
Yes, despite being from a 9 speed road groupset it will theoretically work. If fact any Shimano road or MTB shifter for 9 speeds or below would theoretically work. The reason is that for 9 speed and below derailleurs, Shimano used the same cable actuation ratio - the ratio of cable length pulled to cage movement.