I have also seen in Berlin a big tank with twoe wels around 1,50 m, some sturgeons and big pikes, and they did well too. I think this can work if the wels have always enough to eat, but I also read in german forums about people who had smaller wels in their tanks, that they ate or attacked other fish.
Just look at this video, I suppose the fish the wels eats were to feed it, but I suppose it would do the same with other fish too:
http://www.youtube.com/watch?v=7_v6fOLwbus&feature=related
A further problem is that a wels which is kept in a big pond in tropical climate would probably show never-seen fast growth. Even in the warm regions of Italy and Spain where thy were introduced they have to slow down their metabolisms during winter, but in a region where they can eat and maintain a high metabolism, they could probably reach a length of 1,5 m + in a really short time, i.e. within some years.