I see nothing wrong with mixing your own liquid, as long as you do know what you're doing!
Of course it's very important to have a clean environment when you mix your juice.
But AFAIK PG (and also VG) is bacteriostatic, so even if some bacteria would go into the liquid it would not survive long there.
Another thing to keep in mind:
Nicotine is not a toy! So get to know how to properly handle it before starting to mix.
Apart from that:
Hayppy mixing!