Is it ok to take dayquil/nyquil though i'm about to start a prevpac (lansoprazole amoxicillin and clarithromycin) (a ppi, and two abs) for an h. pylori infection?

Probably safe: I am not aware of any major untoward side effect of using nyquil or dayquil with the prevpac (lansoprazole amoxicillin and clarithromycin). I would not take it if you don't have to. These are usually meds for some symptomatic relief of a common cole with congestion. You don't have to have this. Be judicious. Weight benefit versus risks. Risks appear minimal to me.

Yes: Unless allergic to it, should be fine