LOL - never ask another reefer "Do you think I should upgrade?"
You should ALWAYS upgrade!!!
Seriously though - you have to look at the metabolism of each fish in the system - I had 3 fish at one time in my nano (7g) and I was fine, they were all "light" swimmers, blennies, gobies, etc - and one firefish.
But if you have highly active fish in your tanks, you're going to run into problems no matter what.
However, the best way I've found to figure out if you're overloading is to look at your tank - what is it telling you? Do you have Cyano issues? Do you struggle with other algae?
Also - if you worry, do more frequent water changes, that'll help remove the crap and keep it at a normal level of being.