I heard that MTS multiply like crazy, but you never know it because they stay under the gravel/sand during the day. So you could potentially have 100's of MTS in that tank size in a short amount of time but you may only see a few, which means you have a TON of them hiding in the gravel and pooping, adding a huge bioload.
They can be beneficial in planted tanks, though, because they aerate the soil and prevent dangerous gas pockets. I'm sure I have tons in my 55 gallon planted tank, but I only ever see one or two of them during the day. But one time I turned on the lights after it had been completely dark for a few hours, and a whole bunch of them were climbing the glass and poking around on the sand. I'd never seen so many all at the same time.