150mm inside 200mm

hi again.
This is a continuous from a question that i made in a post.
like i said, i have to put a 150mm pipe inside 200mm.
The question is if i need to go on over the 200 mm pipe, or its enough if i go about 6m inside this. At the end of the 200 pipe it begins a mansory chimeney, about 2m more
Ah, another thing, how would you seal it in the entrance to the celing. stove would be just below and pipe would go straight up.
thats the best idea i have, althou id like to locate just in the corner, the 2 45* pieces would be hanging literaly and i don´t know how to fix them