Do not put the betta in a one gallon tank. He will be obscenely bored and depressed. Besides, a 1 gallon is - so far as I am concerned - completely inadequate for any fish, especially one used to a great deal of space. Bettas shouldn't be in anything much smaller than 5 gallons, with 2.5 being a minimum. After all, bettas like to have plants, caves, AND swimming space; where are you to fit that in a dinky one gallon? Not many people would take an eagle used to flying free outside and put him in a parakeet cage and consider it "ok" just because he could survive in it. You've done great by your betta so far; please keep up the good work
If you put him in a 5-10 gallon tank, you could get a small, peaceful fish or two to live with him. Or, you could just put him next door to your fantail goldies so he can see them without being in the water with them. I don't know how big the prev. tank was, but 2 goldfish in 20 gallons is going to get ammonia issues fast; usually, the rule is 20 per goldfish. Maybe in the larger tank the betta didn't have problems, but a smaller tank means more concentrated ammonia.
You could leave him in the orig. tank, but as someone else said, the remaining goldies might behave differently and become agressive towards him when the social order shifts, and the changed number of co-inhabitants might make the tank seem suddenly too big, which could stress him as well.