Sorry, you say you're "looking for advice, not people telling me its a bad idea" but what if the advice is that it is a bad idea and you need to make some changes? I honestly can see where you're coming from, but a lone discus in an 18 gallon tank with sharks and clown loach..?
The research you've done since buying it must've shown you that this isn't appropriate, it just won't work out even for the 6 months you have before you will add another tank. Even if it were to survive for 6 months, when you get a bigger tank, your discus will still be on it's own with pretty incompatible tank mates -sharks are too active, and big, for discus.
To me you're not going to get to enjoy your fish at there best in the current set up. Chances are your discus won't survive for the 10 years or so it should; however long it does last for I would imagine that you will end up with a fish that will not grow properly, will not develop great colouration, will not have that fantastically round shape to it, etc etc.
So, my advice is that you need to make some changes - decide on a bigger tank, rehoming some fish, or preferably both. I know that isn't what you want to hear, but you did ask for peoples advice /information.
In the meantime, keep the water clean. Really clean.
Keep the temp at around 29C for the discus. I don't know if this is OK for sharks or not. If this is going to be a problem for the sharks, then one or the other will have to go. I think clown loaches are fine in this temp range, but have never kept them myself, you might want to check for yourself.
Feed a good mixture of quality food, and watch carefully that the discus gets his share - they are slow eaters, and sharks are not, so this will be likely to give you a problem.