Would recommend Microsoft Teams for private chat with people outside of your organisation. It’s designed for IM. Yammer is more designed for one to many broadcast communications - a corporate message board so to speak.
Hope that makes sense and helps to answer your question!
Yes you can. They have an inbox in the EN. Regarding Teams vs Yammer. Microsoft acquired Yammer back when they didn't have Teams. Understand that Microsoft is positioning Yammer as something for the "outer circle", but this is only marketing imho. Yammer was designed as a stand alone application and has always included private messages/groups etc. Microsoft simply has two applications with a lot of overlap. Yammer makes more sense for communities because it's less overwhelming and it includes an algorithm to suggest conversations, groups, people to follow etc.