I've heard 1 per 5 gallons as a loose rule. Many factors play into this. Here's an excerpt from Ophiuria that explains it:
There is no set number because it depends on many many variables...the type of fish, the "footprint" of the tank (eg, a standard 120g is a 4' tank - not great for tangs - but a 125 is typically longer and suitable). It depends on the temperment of the fish - are they aggressive and territorial? Some of the smallest fish have the biggest attitudes. Are they "big eaters" like some aggressive fish, producing a lot of waste? And do you have things like corals...coral tanks, regardless of whether you keep hard or soft corals, tend to have a significantly lower bioload and number of fish than the tank can in theory hold.
So, the answer is that there is no set answer or guideline. What is important is researching the fish, their behavior and adult size of the fish. Stocking slowly, feeding a reasonable amount, and staying up on water parameters and water changes will help. Your overall stocking will come down to water parameters, behavior and personal appeal of stock size.
As for other animals, in theory, yes, anything that consumes food produces waste and has an impact on your bioload. How much that actually impacts things overall is relatively less than fish. Corals are often fed, impacting water quality significantly.
