I am a New Member at Similar Worlds

If you don't just say 'Hi' and don't say something that indicates you got lovin' on your mind...most will either not answer at all or reply courteously in the same fashion.

Different stage, same basic rules.

I would approach in a respectful, friendly manner. Read their About Me for clues. And never assume people are ignoring you or owe you time. Act as if you are meeting a new pitential friend. And wait for a response. Do not badger.

Depends on the person.

I had one person in particular get cheesed off with me in a PM despite the fact that we had actually interacted on a thread.

Generally speaking, it's best to respect boundaries. But, so long as you are capable of not invading someone's territory, I don't think there are any strict rules to follow.

If their profile says, "I do not answer PM" then do not send.
Otherwise it would be okay. Do not be offended if no reply is forthcoming. Do not continue to send messages if you get no reply. That would be considered freak worthy.

I think it's better to engage someone in a public post first, start a conversation, and depending on how well it goes, then PM them later. I feel it's more comfortable for the other person to first get a vibe from you as to what you are like in a space where you are both talking in public... so they will have some idea of who you are, at least in terms of an ice breaker, when you talk to them in private afterwards. It feels more considerate, imo.

It's the manner in which you approach. Be a gentleman at all times when diving in the PM.

It is usually ok to say hello. Its not ok to introduce yourself with a naughty pic. Use your manners and you will be ok.