As a general rule for most resistance training, a 2-1-2 tempo is a good place to start. That's 2 seconds down, 1 second at mid-point, and 2 seconds back up.
Some people can do squats everyday. Giving yourself a full days rest though between weighted squat days is a good idea though. Also, it might be a good idea to mix in some other lower body exercises (dead lifts, split squats, lunges) to prevent acclimation and plateauing.
Squatting to 90 degrees is the ideal minimum range of motion. However, depending on what you are training for, a deeper squat may be appropriate. That really is an activity-specific item.
As for base of support concerns, you should be carrying your weight across the entire foot with your center of gravity typically just forward of the shin bone. Feet should be shoulder width (or slightly greater) apart, and your toes should point forward throughout the entire lift. How you move your hips during a squat is far more important to glut activation than how you are carrying your weight through your feet.
A couple major safety/form items include:
*When squatting, draw the hips back and keep your knees over or behind your toes. This effectively targets the gluts while preventing an unnecessary forward load force through the back of the patella.
*Start out in front of a mirror, and watch yourself at the knees. If your knees are buckling inward or outward then you are dealing with a biomechanical inefficiency that will likely cause problems down the road. Throughout the squat, you want to keep your hips over your knees over your toes (or slightly inside your toes if you choose a wider base). If your knees buckle inwards, your gluts are probably either tight and inhibited, or weak.
Hope this helps.