Bakance charging is dynamic - cells may not be balanced when you interrupt a charge like that, but they should be pretty close so I don't see anything negative from doing it.

Fast charge mode for a lipo just eliminates the CV part of the charge sequence (charges at constant current (1C or etc) followed by the constant voltage part, CC/CV as its labeled). The CC part puts about 90% of the charge in and takes the smaller amount of time, maybe 20min depending on lipo capacity and charge rate, 1C or 2C, etc. You should try it and see if you have enough time for it to finish.