Why 两个 is correct but 两 is not?
Is it because you need to explicitly tell thst you are referring to individuals (doing it with 个), or am i wrong?
Talking about quantity you will always need 个or a similar word, 只for animals, money is another one. That's why you say 两块.
Would it also be acceptable to write 两口弟弟？
Never heard that before. I would tend to think no.
I wouldn't use it, only when referring to the whole family, or the whole population.
No it would be false.. You only use "kou" if you re talking about some people.. However, if you re talking about your little brother you use ge.
I get we're supposed to use 两个, but wouldn't "李老师有二个弟弟" also be acceptable?
It is not correct to use 二 with measure words
But then why is it 二元 (or 二块) and not 两元？