Hello and welcome to the forum. Congratulation on wanting a Macaw to join your flock.
Macaws are very different in personality, behavior,diet and care then Greys.
There are many reasons why a Macaw might end up at a rescue. I know of some Macaws who came from horrible backrounds and have taken years to overcome before the owner can handle them and gain their trust, while other rescue Macaws I know end up in rescues due bad heath issue of the former owner and had to give them up but the Macaws are perfect, happy and healthy. So it depends on the Macaw and their backround.
Purchasing a baby macaw does have it's advantages, one, you are it's first real owner so the bond will happen over a shorter timeframe. How the Macaw matures and it's behavior is controlled by you so you have a direct effect on it's up bringing.
I would advise you to go to a local rescue to see first hand what it available, learn to handle the Macaw and learn it's backround. Handling a Macaw is very different then the Greys. Greys tend to hang out on the owner with very little fuss while a Macaw is high energy, uses their owner as their personal Gym. They are truely feather can openers so watching out for the massive beaks is very important and they can pack a serious bite.
I too would advise you to visit a Macaw breeder and learn everything you can about the Macaw before bringing them into the home.
If or when you bring the Macaw home, you must keep them seperate from your Greys for up to 30 days so make sure you have a seperate room set up prior.
Schedule an Aviary Vet visit for either the rescue or new baby macaw before bringing it into the home. Have the Vet perform a complete physical including blood test, this way, you ensure that you have a happy healthy Macaw and will not pass anything to your Greys.
one more thing you must know about Macaws and greys, Macaws do not do very well around Greys or Parrots that give off powder or dander. Macaws can get very sick. Please make sure you have some sort of air filtering system close to the Greys cage if the Macaw and greys will be in the same room together. Also, your Greys will need more baths or spray showers to keep the dust down.
best of luck on whatever you choose and please post lots of pictures as all us members love to see them. Thanks Joe