I would for sure rethink the mandarin. They are notoriously hard to keep. I purchased an ORA tank raised one a few weeks ago and she isn't eating as much as she should be. She was on pellets for her life and now won't touch them. lol They are know to revert back to pods
frequently and I know that new 14g tank doesnt have enough to sustain one. Just some food for thought.
Also a 14g is a bit too small for them, they can grow to a few inches.
Have you looked at blenny's and maybe a royal gramma? Gramma is a very colorful fish and I think he would be ok in there. With a 14g tank you are kinda limited as to what you can put in. Or maybe a yellow wathcman goby, nie and colorful and he hangs out on the bottom, I have one in my cube.