They do feel the heat and can't cope with extremes of weather.
My pigs when I was a child were in Scotland, so overheating was rarely an issue 
but we did have a couple of Himmy pink eyed so we had to provide more protection and shade.
Some things I noticed the last couple of years though with the boars (Essex -so warm )
Not too much hay. Enough for one day worth of eating and changed daily. It gets wet (pee) and you run the risk of FlyStrike.
Mine would bury themselves in hay if they felt uncovered, so lots of boxes (DH made a screen lid with layers of flymesh to go on the haybox. They have the darkness they crave , but it's airy)
Have you got any ceramic floor tiles you could put in the hutch? You'll need newspaper on top or something absorbant - maybe fleece?
Plastic bottle of water ( 2 litre soft drinks bottle) frozen and wrapped in newspaper, put in their bedding.
Out in the run, a shade (I use an old dark duvet cover)
Or you can wring out a towel in cold water and drape it over (wring out well , they won't appreciate a shower )
Lots of salady veg, but always water on offer.
We have a removeable window (inside is re-inforced against foxes and flies) a fan on the way and a thermometer. They get a nice breeze through but the window is high enough not to give them a draught.
Read up on the signs of heatstroke on guineas.
Most of the time, if they are lardy-ing about, mine sit like a croissant, and if we go near, they move quickly.
A heatstroked pig is flat out, either on their side or stomach. Rapid shallow breathing. It can kill them, you need to wrap in a cold wet towel to revive them then monitor (they might need rehydrated , might need the vet )
Like many pig related illnesses , there's alot you can do to prevent heat related problems.
I always feel I can cope better with keeping my pigs warm in winter than cool in summer, yes even at 6.30am when I'm heating up the Pighouse ready for my boars to go in.