Is there a work around for contacting sitters when puppy is too young to be "fixed"?

Rover Customer Support would be best able to answer and help you- call them at [Edit: Rover’s contact options have changed. Visit the Rover Help Center at https://support.rover.com/ to find the phone number, help articles, or chat with the team]

I mean they should take them in, unless they do not take in unspayed/unneutered puppies. You can offer to pay more, as a last resort. Unless you can find someone who cares for unspayed/unneutered puppies and dogs.