People do it successfully all the time. I don't do duck eggs, I don't know if I'd say "better or not". I would think it would depend on how much of a size difference there was. A good size set of standard eggs I would think would be perfectly fine incubating together with the ducks, however I wouldn't try incubating both duck and smaller.,especially bantam sized eggs together because of the difference of rate of moisture loss. But, if you are going to incubate them together, I would set them so that they are hatching at the same time.
I raised an only chick and the challenge wasn't the chick learning to do things for himself, the challenge was him being by himself. A only chick can be a lonely chick and a lonely chick can be a very LOUD chick...lol Needless to say, he was out with us a good share of the day and I had to move his brooder cage beside m bed so I could get sleep at night. You can put a small mirror and a small stuffed toy in there as "company", but these didn't help with me. The best advice I can give is to see if you can pick up a couple friends his age to put in with him and keep him company.
As long as you see veining that's the main thing. Sometimes you can't see the growth as well because the little embryo is in the middle of the egg and not out against the shell as it is in the first week.
Sorry to hear that!