I think it's quite hard to get the information you're looking for from "practicing smallholders today"! I suspect that very few smallholders keep a house cow - now there will be loads of responses to prove me wrong! I've kicked the idea around too, in an abstract way, so I will be really interested to see what advice you get.
Apart from not having enough land at the moment, the two reasons we don't have a cow is 1) it would be difficult to get a minder if we did want to go away (throwing a bucket of pellets to a couple of pigs is quite different to milking once or twice a day) and 2) the time it takes to process the milk. I'd like to have a go at dairy processing though.
Seymour recommends Jersey cows as being small, easy to handle and giving high quality milk ie high butterfat. Downside is that the carcase of any bulls is inferior for beef production. The same would be true of any extreme dairy breed, though. If you didn't want a replacement for the cow from a calving, though, you could cross to a beef bull which would help.
You could look at Shetlands or Dexters - they seem to be popular with smallholders. I did ask a breeder of Highland cows if they could be milked or would they just kick me into the middle of next week - and they said that Highlands used to be treated as dual purpose in the Highlands but were now regarded as a beef breed. However, you could milk them - milk of a high quality buut not a huge amount although additional feeding would increase yield.
I wondered of you could milk the cow once a day and then put the calf back in, but I can't work out the logistics.
I haven't even seen a book about house cows. My auntie and uncle had one - my uncle was a shepherd. I can remember the big pail of milk in the pantry and it being skimmed for the cream. Wish they were around now so I could pick their brains.
Sorry to ramble on - hope someone has something sensible to add.