Are either of your two big ducks Males?
I had a swedish drake in addition to my call ducks (he had three sedish girls of his own) and he tried to mate my call duck girls - and he was MUCH bigger than they are.
I had to get rid of him and his girls because I liked the calls better. Alternatively, you could just keep the two sizes seperate. I didn't have the facilities to do that.
If you have female big ducks then you probably won't have a problem. If you have a male call duck, he'll try to mate with them and he may be succesful
but I doubt it.
Other than the size problem, I adore my call ducks, they are so funny. Not "lap ducks" but they are friendly enough, and they make me laugh. They love to be squirted with the hose, and every time I get the hose out for any reason, they all come running and mob me begging to be sprayed.
eta - no special concerns, they can be treated like big ducks (not necesarily Pond Safe 24/7, but if ya lock them up at night, they can freerange to an extent.) Not sure on life expectancy, never looked it up. They are seasonal layers, but since it got cold so early, one of my females has been laying again - the eggs aren't fertile so far, but the boys are getting that "springtime feeling", too
so they may be soon.
meri