score:4
Everything is possible. Strollers are allowed in the metro.
But I would advise against it because there are too many stairs in the Paris metro that could make your rides far from enjoyable.
If you read french, here is a blog post on the difficulties of a young mother with a stroller: R.A.T.P : Rame Avec Ta Poussette ! The article is relatively old, but situation has not evolved since.