I don't know how true it still is, but my impression in ages past was that the pachanoid and peruvianoid (especially the macrogonus) types are notorious for the "pot of snot" but that T. bridgesii (aka Echinopsis langeniformis) tends to have less mucilage.
Some of the old extraction studies used the greener outer layer skins of the cacti, not because they contained the alkaloid and the core did not, but because the core was super slimey and it made extraction of the alkaloids too challenging at the time, so the outer green layer was used and found to contain 4%+ mescaline in that study, or so is my understanding.