New dog with old dog and cat

Question
Thank you for any input into this!  I just adopted a new dog,(Mo)Chihuahua/Dachshund mix, male, 2 yrs, 10 lbs.  I have another dog,(Moxie)female,pit bull/lab/Dachshund/Chihuahua mix (?), 4 yrs old, 36 lbs. and a cat(Mocha), female, Maine coon/Tabby mix, 5 yrs old, 8 lbs.  The two dogs have been getting along fine with the exception of the older dog growling when the younger one was trying to walk on her when she was sleeping.  She has not done that since.  Ever since I have brought the new dog home, the old dog attacks the cat when she comes anywhere near either of them.  I just wanted to know if there was anything that I could do to have the old dog stop doing this so that we can all live together in harmony.  The new dog does nothing when this happens.  I don't think he knows what the cat is.  The cat has hissed at the new dog before, but even when she doesn't hiss, the old dog still attacks her (jumps on her, paws at her face, runs after her).  Any advice would be wonderful!!  Thank you!!

Answer
Your "old" dog may be attempting to control the cat's interaction with the "new" dog, in order to establish a rank relationship with the new dog, and (also highly likely) to make a statement to the cat regarding HER job as "leader" (i.e., don't interfere, cat, can't you see I'm doing something important?)  Your old dog may ALSO be observing subtle body signals that you do not perceive in the new dog, who may have a great deal more interest in that cat than you think!  (The cat appears to think so, too.)  This entire relationship is brand new.  I understand your anxiety regarding it, but it's premature.  Give it two weeks to resolve into a more daily routine.  Meanwhile, so long as neither dog goes AFTER the cat to harm it, and so long as the cat isn't obtaining a serious fear response to her former companion (the old dog), do what comes naturally.  REACT.  In a snarly voice, say "Nah!" to interrupt any behavior of the old dog toward the cat.  Observe the new dog: this is a honeymoon period and your new dog is a Doxie mix with a very high inherited prey drive who may decide the cat looks very interesting.  Put the new dog on long, lightweight nylon leash for a week or so; this will prevent the dog from getting into trouble, give a clear signal to the older dog who is who here, and also give a clear signal to the cat that the new dog can't get too close.  Repost in two weeks with update.