I've only used gambusa, which are in my memory quite small. Los Angeles County used to give then out for free, back then, anyway. I used to design small ponds in concrete "pots" with pumps and small fountain fixtures (some of the pots fancy and fairly large), and had one myself, and I used the gambusa myself and mentioned them to clients. We designed much larger inground ponds, but that's another bag of beans.
I haven't read up on all that since, but can imagine if one has a larger pond, larger fish could be useful. Do they use young tilapia for it? I'd guess a juvenile tilapia is still larger than gambusa, but don't know that.
I have a photo somewhere of a heron, if I remember, a great blue, landing on my 'pond'.. but the fish had places to hide in with the water hyacinths. Plus, the heron saw me and backed off.