I have 4 heavily pregnant guinea pigs and are almost sure they have mange mites, i.e. excessive hair loss, scratching, biting etc..

I have brought a parasite spot on for mange mites which contains invermectin, is this safe to administer as there is nothing mentioned on the instruction leaflet about animals being pregnant

The only way to know for sure if it is mange is a trip to the vet.
Lice can also cause some of the same signs, hair loss and itchiness, etc.
In which case a lice killing shampoo will take car of the culprit.
I am not saying it is one or the other, I don't know, but in order to provide the right treatment you need to be sure of the parasite.
While the Ivermectin is usually safe there may be other ingredients in the topical treatment that may be harmful.
I would call my vet and ask or better yet have at least Guinea Pig checked, odds are that whatever is affecting one is the same thing that is affecting them all.