Wow they're really that small? they look a lot bigger in the pic, i.e.4-5inches
But if they really are that small then i doubt they are a pair, if so then once they do indeed get to 5-6 inches and mature you may find one gets attacked.
I know nelly has one in with his Oscars and I suppose you may find one GT will get on with your existing tankmates but they only real way of finding out if it works is to try it. Personally I wouldn't really be keeping two in one tank unless they were in a species tank and were an established pair.
Tanksize for an established pair is going to be 55USG at the very least, if they aren't a pair then even in a 100USG you can't be guaranteed that they would get on once they matured
Edit: you may need to watch the Oscar with the Clowns, Oscars grow at a rate of about 1inch per month whereas my clown loach have grown at about 1inch per 6-8months over the past year and a half, if the new clowns are too small, the Oscar may well get big enough to consider them as food