E.g. Start off with Lair of the Wyrm, then continue to The Trollfens?
Would it be viable to combine two small box expansions to make a large campaign?
Probably not since the quests are designed as act I quests. So some quests might be broken if the heroes have some placement skills.
Not sure if you know but that's what Mists of Bilehall and Chains that Rust does. Together they form a full campaign and I guess you could incorporate one of those other small box quests as a side quest via rumor card. But most of the small boxes are side or stand alone only