Does depend on the map size, difficulty and of course your personal gameplay.
Co-op is not really faster as singleplayer except you play with people that play more aggressivaly and attack the AI early on. Than the game might llast not as long as a single player campaign.
You sould except 10 hours for a normal game. It might take more or it might take less, but I think 10 hours is a good point where to start.
However, like I said before, it depends on the size of your map. If you start with 20 planets the game won't last long but on the other side, playing with 20 planets is already hard as singleplayer, now imagine this in co-op where you have to split up the numper of planets towards the players.
You should start in co-op with 80 planets (default) to see how good you can be. If you notice that you have diificulties to split up the planets equally among your friends you should choose a bigger map but on 8ß planets this won't be actually an issue.
Expansions and minor factions should depend on what expansions the other players have. You will of course find more players with only the base game but you can for an example use ancient shadows, so one player can take the role of a champion where he only controls a champion ship without building a fleet. Thats for people that prefer a RPG experience over a strategic experience.
You shouldn't probably put all minor factions up if you play with players that you don't know well. It might scare them away if they never handeled for example devourer golems.
You also should ask if people want to play Fallen Spire. Since this is an entire story intigrated in normal game it affects the game heavily if you decide to finish the campaign. It also forces you to do some steps that you might have skipped (like conquering specific planets).
