I just moved my betta into a tank with endlers. What I did to see how he reacted was floated him in a cup inside the tank while I cleaned the quarantine tank. As I watched he was utterly uninterested in the little fishies.
I think everybody says that it depends on betta temperament, the problem is how to determine that. So far floating the betta (or the new guy) in the tank and observing behaviours during that time can be a good clue. If he's flaring like mad with a cup in the way, you can bet he will probably attack if it isn't there.
That's an idea to try if you want to add anything else.
My 20 gallon community: 2 Mystery Snails - Arnold Sluggenator & Goldie Brawn, 1 Zebra Nerite Snail - Poop (my pet name for my boyfriend), 1 African Dwarf Frog Juan - Frog, 9 Male Endler's Livebearers - Cloudy, Clyde, Conga, Darius, Frank, Lemon, O'Rien, Peephole, Urkel & Various live plants - anacharis, anubias, hornwort, java fern, java moss, narrow leaf chain sword, water lily, water onion. The Prize Jewel - Dragonscale Crown Tail Betta - Sylvester "Sly" Capone