if you really want a shoal of natts, i would suggest looking into a 75 gallon tank, if you can't go that big, i think you'd be better off with a solo serra such as a sanchezi, mac, eigenmanni, or irritans.
if you do end up getting the natts, i think that buying a 29 to start them off in would be pointless, they would outgrow that tank in a few weeks... you'd be better off throwing them into their permanent tank right off the bat.