I'm not sure how you would take it; but I seldom take many minerals without food. It makes me dizzy and sometimes nauseous. Having said that, I can say that I have taken different forms of magnesium right before bed on an empty stomach. This is in hopes that it will give me bowel movements by morning. Maybe I just don't realize my sensitivity while I'm sleeping. I've even used the citrate form for this purpose. I'd be fine with some liquification (is that a word) once in a while. Sometimes I pray for it.
Citrate is a good choice for you. Magnesium citrate has been found to be one of the most bioavailable forms of magnesium. The more bioavailable magnesium is, the less you will have problems with BMs.