How does poison work in Commander?
The Comprehensive Rules cover Commander rules (903) in about a page. It only lists one additional rule for winning/losing the game.
Yes, 10 poison counters kills you in Commander.
No, poison damage doesn't count double, Commanders that deal poison damage, deal damage as any other poison creature would deal damage.

903.13. The Commander variant includes the following specification for winning and losing the game. All other rules for winning and losing the game also apply. (See rule 104.)
903.13a A player that’s been dealt 21 or more combat damage by the same commander over the
course of the game loses the game. (This is a state-based action. See rule 704.)