I suppose I didn't address the original question, you shouldn't be looking for new livestock. You are pretty well maxed/over stocked. The eel and damsels are ok, the trigger adds a lot of stress to the bio filtration because they are "messy" so I wouldn't add anything else. A good way to tell how your system is on bio load is parameters. Do you have perfect parameters? If not, why not? If so, how hard is it to maintain perfect water quality? If you find that your water is perfect and is extremely easy to keep perfect, you probably have plenty of bio capacity remaining. If you water just can't get perfect and your doing all your maintenance, a lot of the time the system is overloaded. It's a lot of trial and error.