Although I have cows and ponies here, I never graze them together....thats not to say it cannot be done though!! I can appreciate that your mini shetlands cannot eat all the grass you have, but my initial thoughts are that a cow on there full time as well, will definitely make a big difference. For one thing, the field will get covered in cow pats - horse piles you can pick up, but not as easy with cows as you will know. The land will get bare, and do you have a plan of what to do with the ponies and cow when it needs resting - as it will do.
If it were me, I would look into splitting the field in half, so that you can alternate the fields, or even keep the cow in one half, and then when she has eaten some grass down, add the shetlands.
I know an acre paddock seems a lot, but a cow do eat a lot of grass, and like I said, it will soon eat the field off. I have had two ponies in an acre paddock for just 10 days, and it is completely bare!! I am fortunate that I do have more land. Thats to give you an example of how quick it can be eaten off.
Hope you can get something sorted - Dexters are really nice cows, we used to have one many years ago.