When I was writing Columbus Day, I used a spreadsheet to keep track of characters, alien species, The Rules, the technology, etc. Expeditionary Force was always planned to be a series, and halfway through writing Columbus Day, I realized I needed to plan the story arc for the entire series so I would know where the story is going.
Back in the day (2004) I began watching the TV show 'Lost'. After investing three seasons, I realized the writers had absolutely no idea where the story was going, were simply making shit up each season, and none of the plotlines needed to make sense or be consistent. To me, that is disrespectful to the fans. ExForce fans can be confident that each book and novella in the series is advancing the plot toward revealing who Skippy is, how he came to be buried in the dirt on Paradise, who threw the planet Newark out of its original orbit, why Elder sites across the galaxy were destroyed, etc.
About 'novellas': The first novella 'Trouble on Paradise' explained what happened to UNEF on that planet after the Merry Band of Pirates left, and the events of TonP drove the action in the subsequent book 'Black Ops'. TonP is a novella around 55,000 words which is long for a novella. The next 'novella' Book5.5 will be around 90,000 words (a typical full-length novel for many writers) and will include Joe, Skippy and the Merry Band of Pirates.