Just use whatever you want,is the way I see it,all this permission crap is for the birds.
For one thing all the textures and meshes are put in the maps,if they are in there I would use them.
But as long as one don't claim to have made the textures or the meshes,then all is cool.
However taking a whole map structure and making it look different then calling it your own is 100% stealing-theft.
Also by not mentioning it in the readme or elseware is also fine,if you did say,"these meshes were from blagh blagh map" too me,it's just being nice,it's not required .
Thats my opinion,if the stuff is out there just use it,and the thing is,is this is one of the only forms of utopia around that is left,if all these people want credit for there texture (unless your nice) then they better go work for the company and have papers drawn up so no one can use there stuff.
It's kinda rediculas for people to be complaining about "you didn't say ,so-and-so ,in the readme" pfffft ,if I had stuff taken from my maps ,wich they wont anyway it would be fine by me,I am under the same roof,because I sent it someware to a site of wich it can be taken,permission or not.
And yes I will add that because it is nice to credit the authors,therefore I would do that,but I damn sure I will never ask permission (unless it's the whole map I wanted to remake).