Is it normal for bottom teeth to come in before top teeth?

Yes. Yes, most of the time, one of the two bottom central incisors come in first. This is not a rule, however, and teeth sometimes have a mind of their own, and I have seen the top incisors or a lateral incisor come in first.
Yes. Most babies start with the bottom teeth, although an occasional baby doesn't read the textbook and starts with the top. You may see drooling or slight fussiness, but as a physiologic process, most babies tolerate it well. Avoid teething gels, which can cause methemoglobinemia. Teething does not cause diarrhea or fever (>101), so if you see these, your child should seek medical evaluation.
Yes. This is completely normal. Usually, the bottom 2 middle teeth come in followed by the top 2 middle ones. The side and back teeth then erupt.
Yes. The bottom middle incisors are almost always the first teeth and usually come in around 7 months old. However it is not a concern if the top teeth come in first. If your child has no teeth at 15 months old, your pediatrician will likely recommend a dental visit.
Yes. I have, in my career, seen almost every pattern imaginable, so yes it is normal for some bottom teeth to come in before the top. It is also normal for some top teeth to come in before the bottom.
Yes. There is no set pattern for infant teeth eruption and no set time frame. Most infants start to have their first tooth erupt between 6-9 months, though some infants do not have a first tooth until 15 months. The first teeth to erupt are usually the lower central incisors, followed by the upper ones, though, not all infants have to follow that pattern. Enjoy this age and take a lot of pictures!
Yes. The first teeth are generally one of the four incisors, either upper or lower.
Yes. For most babies, the bottom ones are the first to erupt, followed by the upper central incisors or the canines.
Yes. The first teeth to appear are usually the lower central incisors (middle front teeth).