Thanks guy for helping me!
So, I included the fancyhdr package as you suggested, and this partially solved the problem.
Now, the page number are bottom centered. But, since the command is after the "\begin{document}" command, roman numbered pages still have the page number on the top right corner. Plus, the first page of each chapter is still not numbered.
I apologize for not including a working example! But this time I did

(see point 1, below).
Now, two options ahead:
(0) Quick-and-dirty-solution: can you guys
suggest me a standard thesis template, maybe easy to use, with chapters organized as single tex files? If that template has the fancyhdr package included, that should work. (My problem is that I found a myriad of thesis templates of other universities, but no one does exactly fit the style requirements of my institution. I couldn't find a "neutral" one.)
(1) I attach here a working example, that's the toy sample available on the internet. Starting from that one, I've been able to modify the cls file by trial and error to comply with the needed style. Except for the page numbers! If any of you good guys can