[Is it possible to use classic avatar heads with mesh bodies? If so, how?]

Yes, you can use the classic avatar head with it . 

 

That body should come with a Body Alpha which needs to be worn to hide the classic avatar but if it also hides the head then you'll need an alpha that just hides the classic body. 

[i cant log in, says unexpected maintenance, whats going on?]

I am guessing they are doing maintenance. the marketplace is also down so i guess we just have to wait it out...  status.secondlifegrid.net