I wouldn't feed them mixed bags myself as they create selective feeding (picking out their favourite bits and leaving the rest, like kids not eating their veg at dinner), which means they won't be getting all the goodness they need. I'm not sure where the vet is getting his info, there are loads of different types of pellet feeds with different ingredients so there isn't just one type. Not sure what he means about seeds?
I feed my girls pellets only, but they have 4 different types of pellets so there's a variety of ingredients. Mostly grasses and hays in them with other bits, alfalfa hay (in Science Selective which I also feed) CAN cause weight gain but if you monitor their weight weekly you can see if it actually affects your piggy. It doesn't seem to for my piggies. You can also weigh out the food and give the correct amount to ensure they don't have too much.
I believe there are a few threads on her about the nutritional values of each pellet but I forget the info myself lol. That should help you.