When writing songs with more than 4 voices, or with chords which contain more than 4 notes, how would harmonic leading rules apply? Cause quite often it won't be possible to avoid parallel motion in this kind of compositions... Which resource could I confer related to this type of writing?
The same principle applies - don't write anything that sticks out like a sore thumb! What WILL stick out depends on the style you're writing in.
To see how to handle more than 4 voices according to the Baroque rule-book you could study Palestrina's works for multiple voices.
Also see how to handle 4-part (mostly) texture while revelling in consecutive 5ths. You still won't find many doubled leading note-tonic lines though. We're still tonal, and those WOULD stick out.
(The whole thing's available on IMSLP. If you don't know it, have a look. It's lovely!
I don't know why you are so fixed on rules like avoiding parallel fifths and concerned with voice leading. If it sounds good you can write what you want (except it is for an examination of your knowledge of counterpoint and these rules).
Especially when you write for 5 voices you shouldn't be afraid of parallels. The rule of indepence of voices and their individuality by counter-movement is always a good plan. It depends in what style you want to write. That's why you should avoid longer passages of parallels of 3rds and 6ths if you don't want to make it sound too folky. And don't forget to set rests in your composition, it is good to have only 2 or 3 voices sometimes or unisono passages!