We had a Lab when I was a child and she was never, ever a problem with the guineas, who were on the kitchen floor in an open-top cage, tho under a counter. I can't say in retrospect how frightened the guineas might have been when Jenny, our Lab, leaned in wagging her tail at them. Our Lab wasn't a trained hunting dog, but historically Labs were used to retrieve animals like ducks shot in the water, without damaging the duck, so there's not such a kill instinct. That seemed to be the case with our Lab, even tho she was never trained to do that. Once she caught a baby magpie and brought it to my mother and dropped it when told to. There were no injuries on the magpie, no blood, nothing. She somehow knew how to carry an animal in her mouth without injuring it. I wouldn't want a guinea pig of mine to be carried by any dog, but saying that even with an animal in her mouth, she didn't physically harm it.
I've heard similar about Golden Retrievers - the hunting instinct has been bred out of them. otoh we had guineas before the Lab, who came as a puppy and grew up with the guineas so to speak whereas your dog, Phoebe, is already there in the house and doesn't 'know' the guineas on face-to-face terms so you can't know if he sees them as pack members, or not.
Is there any way of putting a large board on top of the guinea housing so your dog can't jump in? Wouldn't be very handy as a permanent measure, but if it's just for a cold spell, might work?