If you mean "mixed flock" as in chicks, pullets, layers and roosters - yes! Just provide oyster shell free choice for your girls:)
I don't know much about Ducks, but I "think" some people also use Flock Raiser for them too.
I don't feed layer feed period. I use Flock Raiser or an all flock type feed (18-20% protein) year round. I generally have a mixed flock and don't want to keep switching food - Flock Raiser can be fed from hatch until....If neither one is available or the date is too old for my liking, I feed...
How long have you had them?
Do you provide a source of grit for them?
I do tend to agree with @9SpiceyChickens if possible, switch from Dumor to something like Purina Start N Grow. I will use Dumor IF that's the only thing I can find fresh, I haven't had too many problems, but still feel like...