ok well first off Congrats! it will be so fun having babies!
ok now for the questions. its ok if 2 different age fish breed. nothing will happen to the fry. and second yes they will breed with their own family. this happened with my guppies. it wont cause a problem in the first few generations but im not sure if after 4-5 generations of interbreeding if something will go wrong. so just try to mix up your stock every now and then
im not sure exactly how long it will take for the babies to come but once you can see the "gravid" spot on the female its getting close. i hope you have a great time with your new babies!
oh and make sure that you seperate the pregnant female from the rest when she starts to give birth. once she is done take her away from the babies or she'll eat them.