Can i put ONE in a big tank full of
Goldfish?

"Can" denotes "physically possible" so the answer is "yes." ;-)

Is it a good idea? Probably not.

My betta absolutely hates water surface movement. I imagine putting him in the
goldfish tank would make him incredibly stressed. I added a tiny corner filter
to his tank and he flipped out and hid, refusing to come out (except rushing to
the surface to breathe) until I turned it off a couple of hours later. The same
thing happened when I tried it a second time. Probably it would have been okay
if he'd been around a filter since fry-hood, I don't know...