Mastweiler agression

I have a 1 1/2 yr old female Mastweiler.  Rescued at 3 months old from a family with other dogs and a man who yelled, frightended, and we suspect was abusive to her.  She is finally getting out of habit of hiding from us we get home.  She is over 100 lbs.  Bella is very attached to me, as the one who is always home and feeds her.  She rides in car with me, sleeps with me, wants to be a lap dog.  Very lovable dog with my husband and two kids ages 12 - 14.  We have two cats and a 13 year old Collie.  She is aggressive when I am in car with her, barking at anyone on side of street, etc.  When she is alone in car, she sits quietly observing.  She tenses up when someone other than my my family are in room with her when she has a bone or toy, including my Collie.

 

Bella has just had her second 'heat', lack of a job has prevented me from getting her spayed, until today - she is at vet hospital today! 

 

The past two months she has started to become aggressive with her food bowl and toys.  Mostly with my Collie.  He cannot even walk by room she is in without her growling with toy.  He is arthritic and we suspect a little deaf now, and has suddenly decided to ignore his food at feeding time and approach her.  She growls at him, we shoo him away.

 

However, the last two months she has gotten more aggressive.  She went after him once and my husband pulled her away.  Unfortunately, earlier last week I fed them, walked out of room.  The Collie went by her, she attacked him for several minutes, with me futilely trying to get her off him.  I spent that afternoon at the animal hospital with some bloody wounds to my old Collie.  We have kept them separated for the most part since then.  When food is not involved she wags her tail and wants to go by him.  I get instantly nervous.  I feed them in different rooms, and gate them apart.

 

I am hoping the spaying will relieve some of her aggression?  Also, she goes balistic when strangers come to the door.

 

We never got her formal training, so I know we need to do this.  I don't want to be afraid when my kids have their friends over, or when guests come to our house.  She is fine on walks, which I admit are not frequent.  We have a large fenced yard which I lazily have substituted.

 

Any suggestions on how to curb her aggression?