Fix t[iindex] -> t[index] in comment.
 'In the same idea' is an awkward expression in English, can't explain why.
 'I think 'no types' flows better than 'zero types' (in any case type(s) should be plural). In English zero is plural, one is singular, two or more is plural again. Trying to understand the logic behind English usage is fruitless however.
Again a few minor typos, and rearranged the sentence to improve flow.
than -> as.  I've noticed you switch these occassionally, but for the life of me I can't actually figure out how to explain when to use 'as' vs 'than'. If I weren't English I am sure I would mix them up to. Sorry.
pluralization.
In the same idea -> Likewise + missing 'the' .
I am not so sure about this one.  You say that mixin templates and non-mixin templates are strictly seperated, which implies they can't be used together.  But in the next section you point out that mixins can be used 'in' other templates. So I find the claim they are 'strictly seperated' too strong. If my understanding is correct, then what you wrote previous to this statement seems adequate (about it not being possible to mix in a standard template).
Where did DeveloperStack and PersonStack come from.  Did you mean ExtFriendStack and FriendStack?
Added made.
you -> your
you -> your
isn't -> doesn't
being subtype -> being a subtype
let -> lets
Fixed usage derive(d) in a few places.
I cite it there -> I cite it here
better algorithm -> better algorithms
trough -> through
between to extrema -> between two extrema