So I'm getting ready to start the campaign, and I'm thinking I might have 5-6 players. I've looked into the rules for that and all, but I'd like to extend the campaign. Would I do that by simply adding a story mission and a side mission or by adding two side missions? What are your suggestions?

Are you talking about the Core Game Campaign, or the shorter ones from Twin Shadows or The Bespin Gambit? Currently, my playgroup have begun an extended Bespin campaign, and I just stuck in a few Side Missions between the story missions, reduced the awards from the story missions (to make up for the rewards from playing the side missions) and basically tried to emulate the campaign structure from the Core Game. It seems to be working well...

I personally think the full campaign is already too long at 11-12 missions. If the other side wins a few story missions in a row, it's easy for the rest to become very lopsided. (Giving both sides rewards if the same side loses two consequtive story missions could help a bit.)

The easiest way to extend a mini-campaign is to pick some agenda side mission with not-so-great rewards and play it as an additional intro mission. I have used Back-Room Bargains - the rewards for both sides are not that game-changing. And you can reduce the campaign setup rewards to compensate. See https://www.boardgamegeek.com/thread/1601278/cloud-city-crackdown-swia033/

Another way to extend a mini-campaign is to insert side missions and adjust the rewards from the epilogues accordingly. See https://www.boardgamegeek.com/thread/1507945/rise-and-fall-twin-shadows-swia024-finished-201607 . But pick the side missions more carefully than me so there won't be so many green (ally) side missions played.

The intro approach isn't very good for a full campaign, because side missions and especially agenda side missions are not expected to be played at the beginning of a campaign without XP and credits. Some missions could still work (like Back-Room Bargains), so you could hand-pick, but some would probably be too easy or hard. So, If you absolutely think that you need more missions, I would insert another side mission after the first mission of the campaign. I think two might be too much unless you take a few story missions and adjust the XP rewards. (Reduce the XP reward of the winning side by 1XP and change it to 100 credits per hero /2 influence, respectively.)

Use the mission from the ISB Infiltrators; it provides a reward for whichever side wins.

Since you are just starting the campaign, I would recommend just playing through one full campaign before deciding whether or not you would want to extend the next one. I assume you're talking about the core campaign. Having finished two core campaigns and currently on my third core and first Return to Hoth campaign, the big box campaigns are pretty long as it is. In the groups I've played with, missions take anywhere from two to four hours on average. Given there are minimum 11 missions in the campaign, you're looking at a total of 30 hours of gameplay.

The bigger question is how are you planning on handling the 5-6 players? Do you have 5-6 other players including yourself? If you have 6 total players, I would recommend having two people play the imperial and each of the other four players control one hero each. There is enough to do as the imperials where two players could conceivable play the role without affecting the balance of the campaign. Including extra heroes or allowing a

Use the mission from the ISB Infiltrators; it provides a reward for whichever side wins.

Yes! I'd like to see many more missions like this.