Six weeks is considered too young now, but was the norm 15-20 years ago. Now 8-10 weeks is more the norm. The issues that come up when removing the puppy too early from their littermates are:
1. The most important: starting at about 3 1/2 weeks puppies start playing. Play is an important part of development. The most important part of playing with littermates is puppies use their mouths and learn how to control their bite-called bite inhibition. When puppies bite each other and it hurts they squeal and run away from the biter. Puppies learn how to control their bite this way. If they control their bite, littermate will continue to play with them. If they bite too hard, play ends. Singleton puppies are a nightmare and I've trained a few (only one born in a litter) The singletons have not learned bite inhibition from littermates and they play bite their owners and draw blood, or their new doggy sibling in their new family. This may be why you may have heard puppies taken away too soon become aggressive.
2. When puppies play with littermates this creates social skills. Puppies learn hierarchies. The strongest puppies become the alphas starting as early as 3 weeks of age and the weaker puppies become subordinates. Learning hierarchies is important in deveopment and the longer they are with their littermates the clearer the hierarchies become. Another reason why puppies taken away too soon could possibly become aggressive.
3. Play with littermates creates good coordination which helps in development.
It's not a disaster for a puppy to be taken away at 6 weeks. You just need to do the right things to continue proper development. If you PM me, I will link you to a handout on how to continue to teach your puppy good bite inhibition. I just wrote an article on how to prevent aggressive issues in adult dogs by doing proper socialization when puppies. Many books and articles talk about the importance of socialization, but don't talk about how to do it right, or why it is important. I can link you my article.
In regards to puppies on the bed. If you bought a Golden Retriever I'd say put him in bed with you. You bought an American Bulldog Terrier which is a breed that can be status seeking and challenge you as an alpha, and elevation (being at the same level or higher than you) to a dog is status. My suggestion is to crate train and not allow on the bed until you know his temperament. How I did it with my puppy (now almost 9 yrs ago), was to put the crate by my bed-it is very hard for a puppy to be alone after having 2-11 littermates to sleep with every night. I put a stuffed animal in the crate (later my kitten slept with him in the crate), and if you can, find a way to warm the stuffed animal to feel more like his littermate. Some people use a hot water bladder. You could get down on the floor and sleep with him. BTW, one night or two with you is no big deal, 4 months on the bed with you might be. I've had people allow their puppy to sleep with them and when they are sound alsleep gently slip them into a crate. No different than having your newborn fall asleep in your arms and then put them in the crib.
One hard thing about getting a puppy young is, the rule of thumb is the puppy can hold it's bladder for the number of months old it is. So, you need to get up minimum every two hours to let it out. If a puppy starts to soil it's crate, (or your own bed), it will sensitize to it and not care if it is soiling it, and a crate is a key to housebreaking since a dog naturally does not want to soil it's bed/den. Also it's important to get the right size crate. If too big, it will just go to the other end of the crate to soil, and come back to it's bed. I got my puppy at 7 weeks and I think I went to bed at 11:00, let him out at 2:00, then at 4:00 and my husband got up at 6:00. When he whined, he got let out to potty, and my puppy created that schedule. I can also link you to good articles on housebreaking and crate training. Many of my clients would have a hard time crate training. I'd take the puppy for a three day weekend and their dog would go from hating the crate to loving it so much they wanted to be in it all the time.
Hope this helps.