What other fish I can add to the tank to make it "more lively" ?? I'm thinking silver/black arowanas
Not recommended at all, The species your keeping just aren't compatible with anything. You may want to reconsider snakeheads all together. By the sounds of it your shooting for a mix pred tank.
As previously mentioned snake heads should be kept species only, alone or as a pair. If you want a tank that's full of fish and activity then snakeheads are not what your looking for.
There is one exception to the rule, channa pleurophthalma best kept in groups , quite active swimmers pack hunters and should tolerate tank mates that are too large to be eaten as they arn't overly aggressive. They should not be kept with your current snakeheads, but might be a species to consider switching to so that you can open the doors to other none-snakehead tank mates.
Hope that helps !