Hi FauxFox- are your guineas going to be out all the time?
They need to be protected from draughts and damp and extremes of temperature.
My boars are still out in their Pighouse (my DC old wooden playhouse) and when they come inside for a cuddle I have to keep them out of my son's room because it's too warm and I worry that they'll feel the difference.
WRT food:
no nuts
they mustn't have any sugar (I don't know what a guinea-oatcake would be, but the bird ones are lard/suet or something which wouldn't suit guineas). And oats might fill them up too much to eat their food.
Corn on the cob- mine love. If you can get the ones with leaves, peel them back and they get a green veg too. They eat the 'silks/thready bits' . It's good heating food .Beetroot too but the beetroot leaves are poisonous.
Cress or parsley pots they will wreck and you'll get soil everywhere 
Branches- willow is fine as are fruit trees that don't have a stone. So apple, pear are good (there's loads of others, I'll google them but I've got a willow tree next door and a pear tree in my garden, mine eat the leaves too).
Frozen food is a bit too soft when it's thawed so they won't be impressed. Winter veg (turnip, parsnip, celery,beetroot,carrot, pak choi, kale, greens, sweetcorn,fresh beetroot) will be scoffed.
I give our boys lots of hidey spaces and I vary the boxes .
At the moment they've got their big wooden haybox, a wooden box and two rubber trugs with a door cut in.
I've put their cardboard,rubber mats, newspaper,wooden catlitter,hay in the haybox. Fleece in one trug, naice hay in the other.
They've got Snugglepads and Pet Hotties.
Mine don't tend to sleep together but they'll be in the same house at night.In the day they like peace.
They need a space to get some 'me time'
What about tunnels (Pet Shops have those plastic ones or you can get a wide plastic drainpipe or a carpet tube from carpet shops- like the middle of a toilet roll tube but huge) 
In the winter I'm planning to set their rabbit run in the garage with cardboard and boxes to let them have a run somewhere different.
DD and I took them up to her bedroom for a cuddle, we put a cover down. They walked about 5 steps each then climbed onto our legs for a cuddle. So not much excercise (lazy boars) but TBF they were somewhere different so probably wary.