Same general rules apply: be circumspect, learn as much as you can before actually making contact. A bit harder since a space-faring civilization's going to have decent sensors and will be watching their space, if for no other reason than traffic control. (big snip of generally good ideas)
You pretty much covered it except for one aspect: You aren't going to initiate first contact
inside the target's home system.
Think about it for a second... You're going to show up where they live with all sorts of advanced technologies they can only dream about. They can jump one parsec and you can jump six. They can build ships up to a certain size, you can build vastly larger ones. Your computers are better and faster. Your weapons are presumably far superior. Your power plants, gravitics, sensors, medicines, cocktail umbrellas, whoopee cushions, and
everything else you can name are better than their versions.
And you know where they live while they have no f*cking idea of where you're from.
Yeah, they're going to be really happy to see you, right?
Here's what the Imperium is going to do and this is how Rigel Stardin is going to model it...
The Imperium is going to learn about this inhabited sector via the people already trading, raiding, moving about in it. The whispers, hints, barroom tales, and purchased information is going to let the Imperium identify the most likely targets within the region. Those targets are going to be "shirtsleeve" worlds on which people can live without much technology.
Stellar positions can be mapped from back in the Imperium. Stars with the best chance of hosting a "shirtsleeve" world will be identified and run past the "whispers" the Imperium has been gathering. Long range, deep space, scouts will then be dispatched to check out likely systems from a parsec away or more.
CT's Zhodani
AM had rules for everyday ships detecting gas giants at parsec-plus distances, scouts are going to do better and, IIRC,
MT had rules for it.
Once the host systems are pinpointed, the Imperium's diplomats are
going to let the natives come to them.
Rigel is going to take his sector map and put a penny on every "shirtsleeve" world. Those are the possible homes for all his native cultures/civilizations. He's then going to mark out the
jump1 routes which connect all those worlds.
Tweedledum-2 and Tweedledee-3 have both invented jump drive in the last decade or so and are now eagerly or cautiously or haphazardly or whatever-ly exploring their local neighborhoods. With jump1 drives, they're going to mostly stick to jump1 routes and only make deep space jumps when necessary. Sooner or later ships from both worlds will meet and exchange greetings, information, goods, and/or weapons fire. Whatever happens, both worlds now know the other exists in that direction. Whether a trade route or a patrolled border develops it doesn't matter. The Imperium is going to select a relatively worthless system along that route or border, set up shop, patiently wait for traders and/or warships to pass by, and wave hello.
First Contact has now been made.
It's been made in a place which the Imperium can easily control. It's been made in a place where the locals aren't going to feel too threatened. It's been made out among the stars where the locals have already met other "aliens" and not in the locals' home systems which they might be a little bit anxious about "aliens" visiting.
With their technological edge, the Imperium can easily set up a B5, Meetpoint, trade/diplomatic entrepot in the system while controlling the amount of "contact" occurring at any given moment. The "First Contact" system will also act as a base for the small scouts Rigel's OP spoke about. Those scouts will be buzzing around the local neighborhood watching what Tweeldedum-2 and Tweedledee-3 are up to. They'll also be wroking up information for the next contact mission deeper into the sector.
What the Imperium will not do, that is what the bureaucrats and agencies of the Imperium will not do, is send off Scouting Sam with his best pals Ginger, Wimpy, and Gilligan aboard the good ship
Rocinante to blithely flit about the sector landing whenever and wherever they like to contact whoever and whatever they like while getting their asses shot off and ship stolen.
There will be idiots, both individual and corporate, in the Imperium who will try to do just that, but anyone with two neurons to rub together will not.