Want to become a part time walker but have my own well behaved and friendly dog and I want to bring her along if the client dog is also friendly and comfortable with it. Is that strictly a "NO" or is it open to the clients?
This would be an open conversation with owners. I just transparent with owners when talking to them about having your dog come along on walks. Best practice would be to have the dogs meet before hand, to make sure they get along with one another. Overall it's going to depend on the owner and their dog.
I agree with Heather - definitely run it by the owners first and do a meet and greet before the first walk to see how it goes.
It is normal practice for dogs to be with a pack during walks. Full-time dog walkers can have 4-6 dogs with them at a time from different households. So it is a common practice, but should still be run by the owner first so there is no misunderstanding or any issue.