My advice is this....
take the pacu back to yuor LFS. This may be a fish you like more than the puffer, but the pacu will really get to big for you current tank.
Now about the other fish in the tank.....
If your dempsey's are not full grown yet (which I have to believe is the case) you may want to consider taking all those tropicals (white clouds, tetras and so on) back to the LFS also. When your Dempsey's put on some size you may notice these fish disappearing and your dempsey's getting bigger and fatter if you catch my drift here. These fish IMO are just not good choices as tankmates for Dempsey's. The blue gourami may be alright for awhile, but that to is questionable.
If your interested I would be happy to help give you ideas on compatible tankmates for your Dempsey's...all you have to do is ask my friend.