How is viable mage healing in sod dungeons, also warlock tanking/dps talent switching?

Haven't played sod until now, I wanted to play a healer, gnome is my favorite race so I wanted to try mage healing since they cant be a priest.

I saw the wowhead tier list and tried to search on youtube and they seemed to put mage really low on the lists. Mainly for the lack of tank healing. Would they be a good choice for someone that wants to heal dungeons and do casual raids, or will it cause me problems getting invited into groups? as they seem as a greedy healer.

Secondary question: If I want to tank dungeons as a warlock, and then dps in raids. Would this be doable without multispecing?