I had my dragon betta, Walter, in my 50 with two honeys, Clover and Tupelo, for a few weeks til his 10 was cycled and they all did fine. My betta is very mellow, despite being a "dragon", however.
It really depends on the betta. If he's fiesty or a punk it probably won't work.
"The solution to pollution is dilution. If in doubt, do a few PWCs."
55 gallon, Gus and Fred the ryukins
10 gallon, Bart the Betta