[gmuslera]

You can have multiple secondary servers, what is not a good idea is to have just one, because whatever it happens with it things may fail elsewhere (i.e. mails lost, that may be worse than losing connectivity with your website for a while). And once the zone is setup and the secondary working, you only need to modify things in the primary.

Regarding TTLs, how frequently you modify those records, or how probable is that you have to modify them in a near future? Those times will say to resolvers for much time they could cache that information, and give a time for propagation of a change in that information. With current bandwidths and having slaves you can have relatively frequent updates, but have some margin to have meaningful caches in remote resolvers to speed up access to your sites.