For a first foray into gut the tonic will be fine. I would go with the 16g however. But that is just me. I would echo the comments regarding stringing it at higher tension. 58 - 62. You will not lose any feel for the life of the string job. People say that gut us too expensive - and it is more costly initially. But unless you are a string breaker it lasts significantly longer than any other string type. I think that it actually improves as time goes by and is at its best right before it breaks!
Volkl V 1 Pro. Klip Natural gut @ 58#