Can a personal meeting with a scientist be referenced?

Strictly speaking, yes. The APA style, among others, does have instructions on how to cite private conversations (see https://owl.english.purdue.edu/owl/resource/560/11/ for an example), but in practice I've never seen it been used in psychology (or natural science) papers. This is probably because a personal conversation cannot be easily verified, which makes such citations less useful. One thing you can do is to track down the relevant papers and thank your colleague in the acknowledgement section.

If at all possible, follow Drecate's advice and cite the relevant papers.

If this isn't possible (e.g. a scientist told you about an unpublished observation in their group, like, "Oh yes, we often saw that, but never got around to writing it up."), citing a "personal communication" is appropriate.

HOWEVER, if you do this, the person you are citing should know about it! I would send them a copy of your draft before you submit it. This avoids circumstances where 1) you misunderstood them, and they didn't say what you thought they did, or 2) they thought they were speaking "off the record," and don't want to be cited in support of this point, or 3) you are "spoiling" their result by announcing it before they publish.

Yes, it is possible, and I have done this once or twice. As others have said, it is better to cite a published document if possible, but (Smith, pers. comm.) is sometimes acceptable: typically if it's for a minor point that referees are unlikely to dispute, or for a minor detail of published work that didn't make it into the published version.

Examples:

• "Liu and Smith (2006) used 15mm glass culture tubes for their samples (Liu, pers. comm.)"
• "Environmental managers report that farmers in this region generally do not trust models (Freeman, pers. comm.) so we took a participatory modelling approach to increase trust and transparency, following the advice of Jones (2011)."