The type and age of Belgian Malinois you want to adopt or purchase is a decision that only you can make; and while there are millions of reasons you may have for preferring one age over another, if you have any pets already, you should consider their feelings also before you settle on adopting a Belgian Malinois puppy or adult.
The Belgian Malinois is not a large dog, but he is much larger than some other dog breeds and definitely out ranks cats in size. This can be an issue for your other pets when you are introducing your new Belgian Malinois to your current menagerie. The best way to do this is to begin when the dog is a puppy; this way his size will not intimidate your other pets. Unfortunately, the puppy behaviors of the Belgian Malinois can be a little too much for some pets, too, so there is some benefit to adopting a grown adult Belgian Malinois.
A puppy can be - and ultimately will be - very out of control for the first few weeks you have him until he is old enough to be enrolled in puppy obedience classes or puppy kindergarten. And once he is enrolled, it will take some time before he learns enough to put it into practice on a daily basis. The rambunctious puppy behaviors that will abound while your puppy is learning proper dog etiquette can be very bothersome to another pet, and can even hurt another pet.
The adult Belgian Malinois may frighten a cat or small dog with its size. It also might not be as receptive to new introductions to new animals at an older age. Another problem with the adult Belgian Malinois is his background. If he came from a place where he needed to be rescued because he was abused, he could interpret your smaller dog's jumping or hyper behavior as a threat and become aggressive.
The Belgian Malinois also has a high level of herding instinct. He will often run circles around other pets, trying to herd them to a place where the dog wants them. Another problem that might happen is the Belgian Malinois's need to play; he might be inclined to run after a cat or smaller dog, so the other pet's reaction should be measured. If it is a game, it may be safe to allow the behavior; if the other pet doesn't like it, it needs to be stopped immediately. Most dog trainers will say, however, that any chasing behavior in a dog should be stopped unless the dog is being trained to course a lure. Even in this case, chasing a smaller pet could lead to chasing a child or even you.