You can combine a warpaint with a potion to get the +6 benefit. You can also use a set of armor that adds up to +9 for a stat as well, and even as far as a weapon/shield combo to gain even more.
Potions don’t stack, no. Lost Souls gives +3 on random stats. If you spam it enough you will get +3 to all stats. You can then pop elixirs and you won’t get +6 it will just refresh the timer on the buff.