hi there official figure is 16" but that is wrong in my opinion , mine is 18" ( female - males will be slightly smaller) and have heard of 20" on a number of occasions ,and seen pics of wild caught that certainly look bigger than 20"
they CAN be aggressive but for the main part are not aggressive to anything that they can not eat whole . they are one of the few snakeheads that can be used as a display fish mixed in a comunity with caution . the correct size dat is a pretty safe bet , arownana would be hit and miss but put with a suitable sized aeowana then i would say chances are you would be fine , but remember at 16"+ that fish or any suitable fish of that size can do a LOT of damaged to each other if they decided to.
they only way to know for sure is to try it out and have an option to return any fish if it goes wrong . 9/10 you will be fine with a pleuro. if i was looking to make a stunning display tank the mix you suggest is the mix i would go with , i would use a asian aro (green) as it has a deeper body and would look less likley to the pleuro that is may be able to fit it into its mouth.
pleuro and obscura are the only two snakeheads i would ever suggest to people that they could be mixed. and then you must still know what your doing and choose the fish they go in with very carfully , remember if buying at juvie , the three fish mentioned would not work long term as there growth raths will be very diferant and would allow too big of a size gap to develop
good luck with your mix and let us know how it goes
cheers col