You can turn fans off. Regarding low rpm, it depends on the fan speed. Around 30 - 40% of the original voltage i believe, you may find fans start to cut off and then spin back up to 100% before returning to desired rpm.
On 2350 RPM Fans (SP120) I get between 900 and 1100 depending on voltage fluctuation.
On 1150 RPM Fans (AF140) I can get them at a minimum of 600.