I would think keeping cattle and goats together in the same shed is not possible. My goats lie down a lot during the day to chew their cud, with cattle muck about they would always be messy, and if they are milking goats.... Also the size difference would make it quite dangerous, and also if the goats are horned? They get different feed too?
Probably they would be happy to share a paddock, or rotate if separated by electric tape, but I would advise a separate shed for the goats. Also if you already have cattle for milk (I am not sure of Dexters are kept for milk or meat, sorry don't know much about cattle)?, then maybe meat goats would be the best option?