Help please!! i got a new baby guinea pig and..?

alright so well i have a single male guinea pig. he had 2 wifes before and kids from them both but they all died :/ long story which im not going through anyways now hes single and alone and has been for 1 year. i have talked to my dad about getting him a female since he cant be alone for that long. well he rejected since were traveling on saturday and he said there will be no room and then they gunna have kids and well :/ anyways i was over at my cousins yesterday and they have 2 guinea pigs 1 male and 1 female. the male died and the female was left. they have a fully grown cat too. and they had no idea what to do with her so they gave us the female guinea pig. now i have her and shes kinda small .. like she grew up a bit but shes still small compared to my guinea pig.. and now there both in seperate cages cuz im scared to put them together... and well her cage is wired and there is a flat piece of cardboard. shes kinda peeing now on the cardboard which causes a big mess and we have no other cage to put her in. when can i put her in with my guinea pig so they can have babies? is she too young to have babies if i put them together? will it affect her?