It is not ok to use your shower brush. You need a new one. The one from your shower will be softened from constantly getting wet. A dry skin brush is not soft. One of the good ones is made by Yerba Prima and can be purchased at a health food store. Other people have posted that they got a good brush from Walmart. I have never seen one at the Walmart that I shop at. What ever kind you use it needs to be made of natural bristles. Not a boar bristle brush either. Tampico brushes are great.
No it is not ok to dry skin brush when you are all sweaty. Your skin needs to be dry for this to be effective. Damp or wet skin stretches. It would be fine to use the dry skin brush before you exercise, then take your shower after your exercise.