Hiya,
Don't want to disillusion you - but I don't think you've got enough ground

I was always told 1 cow per acre, and really 1 on her own would be lonely

I know Dexters are smaller, but still need grass, you've got a couple of sheep as well, so might hit problems down the line.
Unless you can get your hands on some more land I wouldn't advise it - but others might disagree. Why not split your land and get some

or geese or expand on your sheep flock ?
Sorry !
HTH
Karen