I feel there are several ways to judge how much load a tank can handle. There is the really simple to see part where the fish bump into each other but most of us have a harder time with deciding how much is okay when we look at the tank/water situation. We often feel there should be some magic number but I feel that is not possible because there are too many variables. The biggest variable is how much do we each want to do to make it work!
We can stock a tub full of fish but it may work us to death! The tank size and filter can be less if we work harder.
But for many adding a small number of loaches other the other fish mentioned is not going to tip it into a crisis IF we are aware that we have moved a step closer to the edge. And that move does take a little effort to take care that we don't let things slip too far.
So add the fish, if it feels right but also be aware and watching for the sudden increase to possibly run too much bioload for the existing bacteria. If it does, be ready to do the required water changes to keep the water right. One way to lessen the risk of a sudden ammonia spike is to add in smaller groups to let the bacteria catch up before adding more.
From there it is often , how much work we want to add to our situation?