This is how I made my guest list. I asked my fiance who he wants there. He said his brother and his brother's wife. I wanted my brother and sister and their families. Then, we made a list of maybes. I called each person. Some committed right away, my brother and sister and my fiance's brother. That's 11 people including 3 kids. Some people would not commit. Many of our family members do not get along. I got some, "I'm not coming if so-and-so is there." Fine. Sorry you can't make it. We ended up with 17 people who were willing to commit. Anyone who was unsupportive, "Why do you have to have it there?" got left off. Anyone who said, "I'm only coming if you pay for my hotel and travel." was left off. Anyone who was happy for us, was supportive, and really wanted to be there is invited.
To answer your second question it is perfectly ok to invite people to the reception only, but it's considered bad etiquette to invite people to the ceremony only. Anyone who loves you will understand if you can't invite them all to the ceremony. Just tell them up front why you made that decision.