[Help] Introducing New Dog to Resident Dog

My parents recently adopted a 2 year old female chihuahua, and currently have an 8 year old female chihuahua. Neither of them are very dog-savvy, so I've offered my help in acclimating New Dog into their household. New Dog seems to have come from a pretty neglectful situation, and previous owner claimed she was rehoming the dog because her other dogs were attacking it. So, not surprisingly, things have not gone so smoothly when they quickly threw her straight into the mix of the house.Things are going okay enough, with the exception of Resident Dog wanting to play and New Dog being too scared and nervous (understandably). A little growling from New Dog and Resident Dog is not picking up on the corrections/warnings. Resident Dog is very friendly and well socialized and frequently plays with a group of about 6 dogs ranging from 5lb. to 30lb., and was previously used to living with another dog in the house. New Dog is the only dog she's ever met smaller than her.They currently have not been separating them, except for when unsupervised, so my plan is to start from there to allow New Dog some time to settle in and get comfortable with her surroundings.Neither is crate trained, so I don't know if introducing a crate on top of the already existing stress would be good for the immediate present. They have a small spare bedroom that I will be fitting with a baby gate, which is something I've seen recommended frequently when introducing a new dog.My question is, how exactly do you handle giving attention between the two in this situation? Do I just rotate which room I'm hanging out in, do I switch out the room the dogs are in, and how often should I have them out together? I've dealt a number of times with introducing a new puppy into a household, but starting with a "fresh slate" puppy seems a bit different than dealing with a young adult dog with an unknown past.Also, I know you're not supposed to discourage growling or other non-reactive warning signals for dogs so that they can properly communicate their boundaries to other humans and dogs. All growls I've seen from New Dog are definitely out of fear and asking Resident Dog to back off, not true aggression. At what point should I be separating them? Both are ridiculously small, and outside of their own safety, I have no qualms in stepping between the two.Lastly, because I feel it will be brought up, neither dogs are spayed. I know it would probably be preferable, but I don't think I would be able to get my parents on board with that idea in the slightest. There is truly 0 risk of a pregnancy. The dogs are far too tiny to ever be left unattended in any outdoor situation (predatory wildlife). Please do not crucify me for this, they are not my dogs and it is out of my hands.I'll have about an hour a day to work with both dogs (and train my parents, lol) as well as an entire uninterrupted week starting next week to stay with them 24/7. Thanks for any help, tips, or suggestions!And of course, the mandatory dog tax. via /r/dogs http://ift.tt/2rxRP2S