I found Gouramis a nuisance for anything similar sized to be added so gave up with them, but i did always use to keep Opalines.
I put a juvi Oscar (as a temp thing) in with my adult opaline, there wasn't much size difference, but the Opaline killed it... then started killing everything else, so i sold him, bought another, and after a few months started attacking stuff all the time, so i gave it away and gave up.
I also had a 9" Kissing Gourami that came with a tank that i bought years ago, his tankmates that also came with it were Neon Tetras! and lots of other small things, he never bothered any of them and looked awesome as a centrepiece fish, I then decided to buy a Gold Severum, so i sold most of the small stuff and i bought a Severum of similar size to the gourami (a 7" Gold Male sev) and the Kisser beat to within a inch of its life.....so i gave it away.
So i think Gouramis do well with things bigger or smaller than it, but anything i have tried of similar size, they become a right pain in the butt.
(oh and my step fatehr had a honey gourami that constantly beat up his 4" Firemouth!!!)
SO if they are big....they tend to be the 'Alpha fish' and make themselves the centrepiece