Shutter slower than video FPS
"If I set it to 1/25 (shutter speed) does that mean absolute maximum (or lowest setting)?"
Yes, if it was a non-digital video camera. This rule applied to the old non-digital (film) video cameras where you cannot set shutter speed lower than the frame rate.

Meaning that one celluloid was exposed for the full 1/25th of a second. Assuming 25fps. 

For digital video cameras each frame can be blended with the other. This technique allows you to film in low light situations at the expense of motion blur. It can be used to create what some called a dream like sequence.

You should just view this as another creative control that you have to managing low light situations or adding motion blur to your videos.

See example below: