Dexters used to have a dodgy reputation for being jumpy, but this has long been bred out.
I wouldnt put any of the continental breeds onto a Jersey or anything else for first time around. Try jersey or old style angus or something wacky like an Irish Moiled! (Which I believe is a fantastic house cow, as like the shetland it is triple purpose.)
I would only use continental breed AI if your first round of AI doesnt work as generally the continentals specifically Belgian Blues, are more fertlie.
Belgian Blue x Jersey or Holstein is the norm in Dairy herds only when the first round of AI doesnt take, (AI only works 40-50% of the time according to official figures) I should know I work on a Holstein/Friesian farm. And have been on a Jersey farm where there were 250 milkers and only between 5 and 10 continental cross bred calves. Says something about the Jersey fertility - very high!
I have no experience of Shetlands unfortunetly.
One thing to keep in mind is that Continental cattle even when crossed are jumpy. BB's are probably the best and seem to be getting better but Charolais, Limousin are jumpy despite the breed societies best efforts.
Cross breeding increases the likelyhood of throwbacks so bringing back really mad cattle you can ask a local farmer who thought it would be a good idea to cross Limousin with Highland. (They broke out and ran 12 miles before being culled via slaughter man and police marksmen on the back of pickup trucks and in helicopters.
So all in all Dexter, Jersey, Shetland and Irish Moiled would be all excellent for a house cow. Just stay british as there's a reason why our breeds are found in a the inhabited continents on Earth.
P.S. I want a Moiley if anyone would give me one I wouldnt say no
P.P.S There are two types of Devon both a shade of Red. South Devons are larger and a gingery red whilst Devon Ruby Reds are dark red. Please state whether they are Ruby Reds or not as to a newbie it wouldnt make sense. I believe you were talking about Devon Ruby Reds. South Devons however are still triple purpose so could probably be added to your list; just to set the Cat amongst the Pigeons.