Ok I don't overstock tanks so you'll have a little room on this but personally I wouldn't put 6 Dwarfs in a tank less than 20 gal, even then you're going to need at least 9 caves and lots of plants, they'll definately start fighting when older.
My puffers have never had a problem with eating trumpets, considering in the wild they'll eat pretty much anything shell based. There have even been cases on here I've seen of them getting through crab shells. Their teeth are designed to break and if they do break unevenly the next time they eat they'll wear them even again.
