Tadashi and Anna are both optimistic, loving and determined individuals. It is likely that the two would bond instantly, with Anna's high-energy and bubbly personality providing a good counter-balance to Tadashi's laid-back gentleness.
Additionally, both characters love and support their siblings above all else, and it is possible that the two could bond by talking about their experiences with Elsa and Hiro respectively. Whenever one is down, the other would encourage them and convince them never to give up on their loved ones. Whether as friends or lovers, the two would get along well.