The Dorado question comes around quite regularly (I asked for opinions not 3 months ago) so it might be worth some searching for more detail. jhowever, these are the headlines that I have gleened from the reading I have done;
Personally, I wouldn't keep one in anything less than a 450g. Considering the size (at least 24" within 2-3 years) and sheer tenacity, you'd be asking for serious trouble trying to keep multiple specimens in anything less than 900g. So many people have tried and watched everything fall to pieces, you'd be incredibly lucky to keep more than one in the same tank peacefully. Of course, there is the point that there is nothing like a Dorado for its energy, even under aquarium conditions. If you have the tank to do it, you're not going to be dissapointed with a Dorado but it is a comitment and you would probably have to stock the tank around the Dorado and be prepared for some losses.
HTH