Question - Why two day iterations and can FAST work with other length iterations?
Short answer - Because I was trying to push the shorter timescale boundary and thought that one day might be too short. And - yes - other length iterations can work also.
Longer answer below...
By choosing a two day iteration you are suggesting that a (dynamically formed) team should be able to complete at least one story in two days. As an agile developer, I would want a story to be roughly that size in any case. Anything larger should be split.
A short iteration with only one or a few stories in a team will encourage swarming and getting it/them to a completed state. Mob programming is encouraged.
Small iterations and stories allows for small releases. I can imagine at the FAST meeting the FAST Project Manager watching the demos and saying to the DevOps team, "Make stories x and y live but not z - that needs some changes now I've seen it".