As far as the 6 week old kitten thing, it's unfortunate, but it's mostly the fault of the people that won't spay or neuter their cats. There are SOOO many cats that need homes and most people give them away at 6 weeks because they want them out asap (i've seen as young as 5 weeks unfortunately), and someone either has to take them, or they go to the pound if the pound even has room, or the kittens are killed.
There are LOTS of people in this area unfortunately that will hit kittens over the head with a hammer or something if they can't find homes and they will tell people so to make them take them. Personally I will not take kittens just because someone threatened to "do away" with them. The fact is, a lot of cats get euthanized even if they are taken to the pound instead, or in the long run even if someone takes them home because they cannot properly deal with an unsocialized 6 week old kitten, so they take them to the pound where they are now unadoptable adults.
I cannot fault someone for taking a 6 week old kitten from an irresponsible owner, unless it is a pure bred and the owner was breeding for profit (in this case people not buying so young would more likely cause them release them later). But most irresponsible owners are simply not spaying or neutering, and whether the person takes them at 6 weeks, is not going to make a difference because the owners just want the cats GONE and will take them to the pound or kill them anyway at 6 weeks if no one claims them.
As far as the cat biting, I think good advice has already been offered, but it's kind of hard to offer great advice without seeing the cat biting and being able to tell if it's strictly just rough play, or it's acting out in some other manner. Obviously the roughness should be discouraged, and IMO the best way is to sit the cat on the ground and ignore it. Only give attention when it behaves appropriately. Even give treats when the cat is behaving calmly. Punishments like pressing on the tongue, hitting, yelling, etc., may either cause distrust, or will actually make the cat think you are playing and soliciting even more rough treatment, so those are a no no.
When you see two kittens playing, you will notice if one is not happy with the behavior they will back off, hiss and avoid. If a kitten bites harder and grabs at the other cat it will often solicit, and is often meant to, an even rougher attack from the other kitten. It says "Oh you wanna play rough! Ok we'll play rough!"
In some cases the opposing cat may get on top of or hold down the rough kitten until they submit, but I don't suggest people try this because they don't generally go about it in the correct manner.
I'm very physical with my kittens (I don't mean by hitting - I mean by handling, play, etc.) in order to teach behavior and boundaries, but I don't do it in a negative manner and a lot of it is actually a trust building thing. I cannot suggest physical means to others to correct their cats, even if it works for me, because I cannot trust the owner to behave properly, gently enough, or for them to have built trust and respect in their kittens prior to using the tactics (and don't misunderstand, I'm never rough or physically hurtful - whereas many people, especially those that are frustrated, may go overboard and not realize it). That's why I think when someone suggests physical means to controlling behavior, even if they are experienced and it has worked for them, often it goes wrong, because there are different dynamics between each owner and cat and differing personalities in each animal (and that actually goes for animals other than cats too). If someone came to my house and handled my cats in the same manner I do there's a good chance it would actually scare them or cause an aggressive reaction because they don't have the same level of trust with that person.
Cats do not build trust and bonds easily, but many of them DO sever bonds and trust easily, so the wrong physical correction can have dire consequences for your relationship.
Point is, if someone suggests physical aversionary tactics like hitting, pressing on tongue, etc., or even more benign things like holding the cat down, they most likely are giving advice that either doesn't work, or don't understand that not all cats and people are the same. Best to stick to advice that is positive reinforcement based.
Oh and I think it's important to keep frustration levels as low as possible. I NEVER get frustrated with my cats (hard to believe, but there's a reason I like raising kittens
), though they do sometimes do things that make me go "What the heck are you doing?". Reason being when humans get frusted they generally act out in a negative manner that cats don't understand. Cats are much more body language oriented than humans, and even other domestic pets like dogs, and our body language and behavior while frustrated often comes off as aggressive, dangerous, or challenging. So try to keep a happy and relaxed demeanor around your cat. It will go a much longer way than anger or lashing out. If a cat does something you dislike, don't get angry as if they were being malicious, try to understand why the behavior occurred and correct it in a way THEY can understand.