Welcome to Aquarium Advice, diggitydawg!!
I don't think your current fish load could tolerate the addition of a loach, unless you trade in the the shark (and the gouramis
but that does not have anything to do with the snail situation).
I have had the best luck with the two-pronged approach. First prong is the romaine lettuce leaf technique, which it sounds like you may have tried, but it has to be done every day for as long as it takes. You ought to have hundreds covering the leaf in the morning unless (second prong of the approach) you are overfeeding. Snails will reproduce and continue to populate as long as there is enough food for them. In the absence of food on the bottom their population will dwindle and fade out. Switch to pellets or a floating food that can be eaten without much waste by your fish, so little to no food gets to the bottom to sit.