I am working hard at keeping a good sustainable pod population in my 13.5g nano. I dosed the tank three times over the past 5 weeks, once with Tiger pods from Reef Nutrition, once with a mixture of Tisbe, Tigriopus and Apocyclops from Florida Reef Labs and once with ApexPods from Reef Nutrition. In all cases I split the dosing over a few days and splitting the dose into my refugium, return chamber and the display. I can literally see hundreds of pods in my tank and refugium. I also have a small pile of rock rubble and a 2"x3"x2" DIY laser cut acrylic pod hotel in the display. With that said, I have had a Mandarin in the tank for the last 3 weeks and she seems to be doing very well. Beginning the first day she has been pecking at the rock and glass constantly, I can occasionally see her actually strike at a moving copepod. IMO, I'm thinking I am close to a stable pod population in the tank. But just in case, I am also running a 5.5g copepod culture tank along with a mason jar phyto culture. The pod culture has just recently started to explode. I feed the pod culture from the phyto culture every other day.
Looking at the glass of the culture tank I counted 47 pods on the glass in a 1" square that I drew on the glass with a marker. I am sure that number is very inaccurate, as I probably counted the same pods multiple times and had pods crawl in and out of the square. Bottom line, that 1" square was filled with pods! I will dose pods out of the culture once or twice a week into my nano just to provide insurance for the mandarin. If you give the pods enough safe areas in the tank and or in a refugium safe from predators, you should be able to generate a pretty good population.
Don't assume that your normal rockwork is safe from predators as I have seen my mandarin squeeze into the tightest of crevices and or go into a rock cave and almost turn herself upside turn to strike at pods on the roof of the cave. Last but not least, with a small tank you don't want to have a mandarin in with any other fish that eat pods.