new dog

hi, we just adopted a dog, and we are a multi pet home. she did meet our senior dog b4 adoption, and they got along fine. it was unknown how she was with cats or any other behavioral isssues she might have, and i was okay with that, as our cats have free range of our home and our dogs stay downstairs. she is unspayed but was put on a waitlist to get spayed. we brought her home and all was well until she started showinf aggression and resource guarding. the resource guarding, i wouldve just trained out of her as i study and train dogs, but the aggression was where it made me nervous.she is a great dog, but is also very clingy and was starting to already resource guard me. we also have a puppy, who is very dog social, but with the new dog, but they both clashed and he gets extremely upset with only her. this has only ever happened once b4 with a different dog we tried to adopt, where my puppy did not like that dog but still tried, it was just a lot of clashing, now its more aggression based. on top of that, our 2 cats, who are extremely confident and love walking around everywhere, now hide and will not go out of their room until the dog is away. i thought it couldve just been because the dog is simply brand new, but they both have never done this with newer animals until her. also, the newer dog is showing aggression towards the older cat (i dont know about the young cat as she was hiding), and thats a big no no, as its a senior cat, and while she can be grumpy, aggression towards her is not something im okay with. it also ended up to where both the newer dog and cat got into a scuffle. is it time to just give the dog back to the shelter? i kept trying to find excuses as to why all of this is happening but its hard to accept when its out of the norm for all our pets to act a certain way. what do i say to the shelter? what if the shelter wont take her back? ive dealt with a shelter before that wouldnt take back a dog we adopted even though i was having health issues while dealing with my puppy and the other dog becoming aggressive towards one another.