Generally male Guineas are protective by nature of their Hens, but the Pheasant may still try to breed the Guinea Hens anyway. If the Hens don't solicit the breeding they will most likely squawk and scream if he grabs them or tries to mount them, and if so I'd have to say the male Guineas would most likely come running and at least chase him off the Hen. I can't say if they'd attack him and run him off tho since I have never had wild Pheasants on my property and I have no first hand experience on how they would interact. You may end up with some hybrids in your hatches tho, lol.
Do you have chicken roosters that try to mount your Guinea Hens? If so what kind of reaction from the male Guineas do you see out of that?