Definitely a good thing you took the goldie back! Otos are fun, though you may see your betta chase them if they're in front of him. They're quick little buggers though, so no worries there! I've got a pair in my 10g with my betta boy, used to have three.
Otos are really finicky when it comes to introducing them to new tanks. Mine was FULL of yummy diatoms; a perfect time to add them! Though they tend to die randomly any time in the first month or so, but after then they're very hardy fish! Many take to zucchini very well, and your betta will probably eat any sinking wafer you put in for them!