I recently picked up Bespin Gambit and noticed, but I'm sure it's present in other Mini Campaigns. The rules state, "In addition to being usable in a full campaign, the missions included in The Bespin Gambit can be played in succession to form a narrative mini-campaign.
Does this mean you can place the mini-campaign inside a full campaign like The Return to Hoth, or is this just referring to the side missions or something else? Any clarification would be great.
No, you can't play The Bespin Gambit as a campaign inside another campaign. This just means that you can string the Bespin Gambit side missions together in a specific order to form a mini campaign. Mini campaigns consist purely of the side missions of their respective small box expansion. You basically play the side missions back-to-back, with a story holding them together. Read "Playing the mini campaign" on page 6 of the Bespin Gambit rulebook, and take a look at the campaign log at the back of the book.
You start with the mission "Reclamation" and go from there until you finish the campaign, which takes considerably less time than a full campaign.