Guinea pigs can definitely get oral thrush, although not sure if they can catch it directly from humans (or how this would happen to be honest).
Oral thrush can be serious in guinea pigs, as it can interfere with their eating and cause other, bigger issues.
If you have any concerns at all that your guinea pig may have oral thrush please have them seen by a vet.
The sooner they receive treatment the easier it will be to avoid complications.