The "inch per gallon" rule is more along the lines of a rough guideline (sounds like a "Pirates of the Caribbean" line, doesn't it?). The main thing is to look at your tank and make sure that the fish have sufficient space to swim and behave naturally, without being crowded or jeopardizing water quality. And to keep in mind that as fish grow, that needs to be taken into account. It is always better to stock for the tank you have now, rather than stocking for a tank you may upgrade to or intending to rehome a fish (especially if you don't have somewhere that will definitely take the fish...).
You could have a fairly small number or type of fish, but if they were all bottom dwellers then that area could easily be overstocked, while the water itself was pristine and there was more room for mid/upper level swimmers. Likewise, just one large territorial fish could claim a certain area (think a red tailed shark claiming the bottom level of that tank), and even though there was more space inch-wise down there it still wouldn't be a good idea to add another bottom dweller.
That being said, if you have just a single clown loach, he would appreciate a friend (assuming he is not himself already too big for that tank). Keep in mind that clown loaches get to around a foot long, so at some point he is going to need a bigger tank, and if you get him a buddy you'll need them both to move to a bigger tank (yours or someone else's). It is up to you whether you'd want to get him a buddy now and potentially have 2 fish to rehome or to require the tank upgrade sooner than expected, or if you want to find him a new home now, and once you have a bigger tank get several new ones then.