a new possibly preg sow to my sow and her 6week old baby ? they live indoors in a ferplast 120, would it be big enough for all of them for now do you think ?
You can, but it would probably be better if you waited until the pregnant girl has had her babies, because the space is RATHER too small.
You will need an extra cage for the mother-to-be when she has her babies, anyway. Long term, you will need a bigger cage for three guinea pigs quite quickly. Plus you need to think over what you are going to do with the babies-to-be...
How far along is she, anyway?
There is a sticky thread with tip on introductions in the reference section.
In this case, it may be worth introducing her to others, as she has a long way to go, and if you are lucky, she won't be pregnant (so chances are that she is).
Is it worth looking into C&C cages, as with two sets of grids you would be quite adaptable for different cage sizes?