Can I install a skylight in the roof with a very small angle, for example, 5 degrees?

You can, but you have to use special design. Usually skylight installed on roofs with a slope of 20 °, but the insulation salary EA enables mounting the window in the roof of 10 °, and the special design EFW increases the angle of the window at 15 ° to the plane of the roof, making use of flat roofs and roofs at an angle from 0 ° to 15 °.