separate that female now!! even if she's not prego yet, she sure will be very very very soon. their gestation is what...(someone verify this) 3 months? plus you really really should NOT handle prego degus, they miscarry easily. separate her now so that if she is prego, you don't do it later and hurt her. plus, you do not want more goos, right? I think they usually have at least 6-7 pups? whoa boy that'd be a busy busy time! lol. there is no "too old to breed", that's just us humans deciding when it's unhealthy to mate. it still won't stop them from doing it. as for the weight issue, what are they eating now? and what were they eating before you got them? goos are cogenitally diabetic so they need a low sugar/no sugar diet. NO seeds, dried fruits, honey, molasses, or alfalfa.
hope i helped