When babies are born, they inherit specific types of antibodies from their mothers, which help fend off different diseases (this is nature's way of protecting babies when they're most vulnerable). Starting around six months of age, these antibodies start to diminish, and almost completely disappear by one year of age. Ideally, during that time, babies are exposed to the "wild" diseases the maternal antibodies had protected them against, and will start to make their own antibodies — the beginning of their immune system.
We give vaccines — which are either parts of the viruses or bacteria (called antigens), or weakened live viruses — to our babies before Mommy's antibodies completely disappear. That's why it seems like babies get so many shots before six months. Another way to think about it: When your baby is five months old and picking everything up and putting it in his mouth, he's already exposed to more antigens than all the shots combined!
As far as a modified immunization schedule, there are many pediatric offices that accommodate parents' requests, but the human body is amazing enough that even a baby's immune system will produce antibodies in response to the antigens in the vaccines without becoming overwhelmed by them. As for whether a modified schedule is safe, the published schedule of immunizations has been tested on tens of thousands of babies with proven immune response, but the altered schedule has no such proven assurance. So I say, follow the schedule and give your baby the vaccinations, as this is the best protection you can give.