Most magnesium products are apt to give you looser stools - they pull extra water into the bowel is how it happens. A chelated type is less likely (the most absorbable chelate I know of is magnesium glycinate). As has been mentioned, starting slow and working up to a higher dose is often suggested on some magnesium supplements and sensitivity varies from person to person. Nighttime is a great time to take mag or calcium since some people's sleep disturbances and insomnia are a result of a deficiency in one, the other, or both of these minerals.

Magnesium is a safe product to take with almost any multi; most multivitamins do not have much in the way of calcium or magnesium (especially a one a day style vitamin), so usually you have to take these minerals separately anyway to get close to the daily % recommended. Many cal/mag supplements, when taken to the specific dose suggested on their labels, will give you at least 500-600mg of magnesium at full dose, so the amount suggested on yours is common and unlikely to be "too much."

"Too much" of a nutrient from supplementation is somewhat rare; the ones to be most careful of are some of the fat soluble ones (K1, vit. A - not from beta carotene - and vit. E) and iron (toxic in high doses, and shouldn't be taken unless you are anemic or told by your physician to take it for whatever reason). For most other things you are pretty safe taking whatever a bottle suggests you to take unless you have some weird medical condition or are taking prescriptions that tell you NOT to take certain supplements with your prescription.