Hmm, a 20 year old system is a bit overkill. An established reef of 1 or more years is a good start. I do agree that you don't want too many pod eaters in the system.
I had my 1 male in a 75 that was 1.5 years old. I upgraded to my 156 two years ago and introduced the female shortly thereafter. I have 35 fish in my system plus 6 cleaner shrimp, 4 peppermint and 1 scarlet and other CUC. I also have a 55 gallon refugium. There's plenty of pods in my system for my two mandarin. If you feed well you'll have enough pods.
Manadarins should not be in a system less than 55 gallons, for sure. My fisrt 2 mandarins, over twenty years ago, were thriving in my 55 gallon with an undergravel filter. I had a gazillion pods in that set up because of that undergravel filter and crushed coral substrate.
There are all different kinds of pods that grow in our systems; copepods, amphipods, etc. Copepods being the smallest and most difficult to see. If you make small piles of rubble rocks in the corners and back of your tank you will eventually see a cloud of copepods hanging over and around the piles. They are very tiny. You will see amphipods and other pods running along the rocks and on the glass. I see them at night. This is when I know I have a good supply of pods.
If you have any doubt then dose your system with pods. Key is to make sure you get a very healthy fish. If they are not eating in the store the stress in change from one system to the other will do them in. This is why you need some experience before you take on caring for these fish.
I have also noticed that they have a really hard time competing with other pod eating fish like certain wrasses and pipefish. Unless you have a really established tank (i.e. PaulB) you will have difficulty keeping multiple fish fighting for the same food source.