It's really not about the tank size, I only have a 50gal tank with 20 gal sump but my Ruby Red Mandarin has lived about 1 year in my tank. She doesn't eat anything except pods but is still nice and fat. I'm actually planning to get a male Ruby Red for her because I think my tank can support another Mandarin.I would love to have a mandarin. Although my tank is well established, it's only a 56g display with a 30g sump, which is not really enough if I can't get the Mandy to eat prepared food.
My secret is that I got rid of my skimmer in my sump so I can use that space to grow chaeto and have a 24 hour refugium. The refugium provides all the pods my Mandarin needs.
Of course, I seeded my refugium with pods and home grew phyto for the first 3 months to establish a very nice pod colony in my tank and refugium.
Don't let your tank size limit you from getting this great fish.
Cheers,
Harry