the negative lead of the power supply, the ground of the output jack, the earth/ground of the boards, they're all the same thing. They should be anyway or you're going to come up against some problems.
If you're not going to go with the opamp method i would recommend sticking something like a 4.7-10K resistor on each input before they come together. Something like this will probably do the job:
I'm not sure what you mean by 'i wire into 1 side with lets say +possi then negg'. The middle pin of the pot goes to your output jack tip, the left or right pin to ground, the remaining pin to the mixed signal from your devices. It doesn't matter which way round the ground and signal are, it'll just change which way the pot turns for volume up and down.
I guess thats what you mean?