Trade fleets are just for show, they are generated when you get close to them. Leaving remnants or pirates in your system can cause the 'trade fleets disrupted' modifier though, so best to take care of them.
7 production can fulfill 7 demand everywhere in the sector, and every demand for lower too. The only reason it wouldn't is if poor access on the producing market is limiting exports or poor access on the demand market is limiting imports, or a shortage has been created through the aforementioned trade fleet disruption.
Multiple same industries just increases your market share which is your 'cut' of the global total demand (this cut is the money your exports then generate). Increasing access, decreasing access on competing markets, or blowing those markets up are also ways to increase your cut. If you check the market on ores though, you'll see its oversaturated as *** and not worth that much anyway because there are only a few refineries creating demand. Fuel and food, though, and organics or refined metals... big markets, not too many competitors.
Location doesn't matter for in-faction import, as long as the access on the colony is high enough. If you want more security on your planets then multiple in the same system makes sense as they will each launch patrols.
Hazard rating affects upkeep cost and also affects immigration/pop growth. Hazard pay is calculated based on hazard rating, too. More hazard = greater pay to convince people to come.
If you want easy, safe credits, then settle worlds you can stick farming on since the Luddic Church is cool with people muscling in on their food production they way other factions really aren't. If you want shipbuilding, find a system with a 150% barren airless world (plus some other planets to stick military bases on) and stick refineries/fuel/heavy industry on it.
