Hello Beppe, I think you have intermediate conditions, and probably neither true lowlanders nor true highlanders will thrive, but I think they all can survive quite well, if individuals are healthy and not too much stressed. 18/20° are sufficient as minimum temperatures for many lowland nepenthes... probably not as cold as needed for some highland (if you grow some extreme ones you will definitely have some problems), but you can try.
Probably some plants will manage to adapt themselves to these conditions, and grow well. Among the species you've mentioned, I think albomarginata, gracilis and maybe rafflesiana could do well; I find that my albomarginata from BE is thriving in every condition, from pure lowland to extreme highland...(now it's into my greenhouse, with temperatures near to 0°.. but it won't stay there any longer )! Where does your clone come from?