Ask the guy with the 27 incher how that fish would do in a 300..... I'd be scared for both the fish and the tank.
This all boils down to personal fishkeeping prefrences wayyyy more then anything else. Some people cram them in whatever they have regardless, others try to provide ample space for any of thier fish.
You would be fine for a while in the 300. If it gets to big or too crazy for the 300 and you get attatched then just build a pond, get a rubbermaid, or a larger tank. You'd have a while to figure it out.
Or like I said earlier, if you like Dorado but don't like max size then just get a shoal of Brycon. Similar attitude, half the size. I believe there's a member over on the other site that has a successful shoal of them...... Might wanna look into those.