Red Tail Shark with Opaline Gouramis?

Provided your tank is large enough they should be OK in the same environment. The basic rules for keeping any fish successfully are :- 1 inch of fish needs at least 1 gallon of water. (3 times this is better). Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every day. If you stick to the above rules, your fish can survive reasonably well provided you also provide them with the correct temperature and other water parameters. If you fail to provide all the above I can guarantee that your fish will be constantly getting sick and will die sooner than it should.