Personally i thnk they should be fine, molly fry are usually bigger than other fry, also 3 1/2 weeks old platys should be able to look after themselves against 1 guppy..
You will probably find the guppy wont even bother them being heavily pregnant it will just want peace.
Just put her in and see how she reacts to them, i have big enlders in with hundres of babies and they not bother them, i have guppies with fry and they dont bother them either, but the odd guppy will be a nusciance, especially if it aint use to being around smaller fish such as fry.. A lot of mine grew up with them so i guess thats a bit diffrent.