As awesome as mandarins are I would not recommend one for a tank your size, we have one at work in our frag tank (100 gallon with 40 gallon sump and its decimated the pod population in that tank- they can quickly starve in smallerr tanks unless your willing to put in the extra work of dosing live pods very consistently. even the captive bred varieties seem to revert to pod hunting over pellets and frozen foods.
Blennys are awesome and a tailspot or others from that group would be an awesome addition. Other awesome fish for a tank your size include clown gobys, neon cleaner gobies, Pink streaked wrasse, possum wrasse, Plectranthias inermis (essentially a tiny peaceful hawkfish)- I'm sure there are many more but those are the first few that come to mind!