Bigger is always better...to a certain degree. There is a point where feeding small fish in a large tank simply becomes impractical. That being said, I know some of the people that make the recommendations on tank sizes...and one of them (an employee of a large scale online fish store) used to work under me when I was 14 at an LFS. That being said...there are VERY FEW SW fish that they will recommend anything less than 30g for. I think this is wrong. Many species given proper care can reach full size in a 10g. (Think about the stocking of large pred tanks...an eel is a a 10ft tank, its a 5ft eel. That gives the eel twice its body length. A full grown clown has more like 6 times its body length in a 10g tank. The clowns are also less messy, creating a lower bio-load. By those standards...a 10g is more that sufficient.)
I recommend pairing clowns when young, because its the best way to do it. If you want to avoid aggression, you want to be able to pair them when the size differential is just enough that one will not be able to put up any fight at all, but still be big enough to take the beating. Most people that pair their own clowns at least buy one that is younger than the other. (Assumed based on size.)
Daily salt checks ARE unnecessary. However, daily top offs are not. I dont know why you would bother testing daily, just make a mark, and fill your tank back up to that point every day. That is the best non-automated way to control salinity, and its what I do with my reef(s).
That being said, properly run, observed, and maintained aquaria shouldnt ever have problems with ammonia, or nitrite. I would only test for these when first setting a tank up, or if I was having a strange problem. Nitrate I only test for if I change a maintenance routine or add livestock. Other than that, test until you have a regimen that keeps your parameters where you want them, and then stick to it.
I would either do a pair of firefish or the clowns...you dont sound too experienced, and even then you wont be able to have constant vigilance over this tank. A pair of both would be too busy in my mind anyways. One or the other...and some inverts.