You actually said 'at least' 12" which to me says 12 maybe bigger. Now you are saying 9-10"? You still can't have measured it if you haven't got a size for it!
Sent from my GT-I9300 using MonsterAquariaNetwork App
In my experience keeping jardini with tank mates is hit and miss...it usually ends bad though unfortunately.
However.... I have found that keeping them in larger tanks can help with aggression issues. So keeping that in mind, I would be aiming for the largest possible tank you can afford/fit in your place. It will also benefit the health of your fish.
I have also found that only larger tank mates should be kept with jardini...so please keep that in mind as well when it comes to selecting the right size tank.