I have several donkeys, a mule and used to have a cob pony.
I believe the cob is very similar to a Shetland in that both will put weight on readily, especially in spring.
So my cob was kept with the donkeys and mule over winter and fed the same, ie ad lib straw, a bit of hay, and a mineral lick. That way he didn't put on weight over winter, but always had plenty to eat. He may in fact may have lost a bit, but he was never hungry and was ok then to graze spring grass without the risk of becoming obese.
Equines are kept warm by the fibre they eat, so however cold it was he always had sufficient fibre available in the straw to keep him warm.
So I would give your shetlands exactly the same as your donkeys, and that way they'll have a bit of leeway to put on weight when spring arrives.