OK, I will start by saying I have only exported animals (rabbits, guinea pigs, and dogs) from a 'clean' country (England) to a 'dirty' country (Switzerland.
'Dirty' simply means that it is not officially rabies free, although rabies is pretty much unheard of these days, certainly in Western Europe.
The UK is one of the toughest to get into becasue of it's clean status (Australia is another tricky one).
In essence you are asking about 2 different trips, so you will need to be fully up to date on the rules involved in bringing guinea pigs from the US to France (I don't imagine it is complicated, but would certainly need to be researched carefully) and then France to the UK.
Going via Paris is unlikely to make a huge difference, as you will be asked about your travel, country of origin, etc. and would need to have paperwork which would certainly show that the guinea pigs had originally come from the US.
So going to Paris won't change the need for your guinea pigs to do 4 months quarantine in the UK.
The only real way to avoid UK quarantine would be to fly to France, and live there with the guinea pigs for 4 months.
Register them with a vet on arrival, and 4 months later you would be eligible for the paperwork needed to enter the UK without any quarantine period.
More information about traveling into the UK with guinea pigs can be found here, but you also need to constantly check this, becasue depending on what happens with Brexit this may all change (and not in a good way) very soon.
Bringing food, animals or plants into the UK