Ok well I am guessing that Teddy is de-sexed since from your post it sounds like he lives with your girl Possum?
As females can get pregnant from around 5 to 6 weeks old and it only takes a second for a male to do the deed, I would say its more than likely she is pregnant.
From your description I would say that she is.
A rat pregnancy only lasts 21 to 23 days from conception. If you dont know when conception was, then you work it out from the first day she not with a male as if no babies arive on day 24 after being away from a male then you know the girl isnt pregnant.
But I would say she is. How old is she? Have you been giving her extra food's?
Billy is a Black Berkshire going by your description.
As Possum is a PEW you wont know exactly what she is under the what masking gene until she has her babies.
If you get babies that have white on their tummy and white feet like Billy plus babies all one colour with no white any where then Possum is a Self.
If you get babies that look like Billy plus hooded babies (a strip or broken strip along the back) then Possum is a Hooded.
If you get babies like Billy, plus some self and some hooded then Possum is a Berkshire like Billy.
If Teddy is desexed so that he cant get Possum pregnant then they could still live together.
Some females get very hormonal when pregnant some dont get it until they have had their babies, then they get very protective and cant be in a cage with any other rat until the babies are 5 weekd old or over, but if she isnt bothered by Teddy he can stay with her.
You just have to watch that he doesnt hurt the babies.
If he hasnt been desexed, then I wouldnt keep their cages to close together as rats can breed through the bars of cages if they had a mind to.
Females can get pregnant again within 24 hours of giving birth.
Females also come into heat every 4 to 5 days and as I said it only takes a second for a male to do the deed.
I hope to hear how Possum goes so keep us updated.