I choose to size skins at least as wide as the tail width (usually 10-15mm less than the tip width). If you have too much base exposed near the edges it may be hard to traverse slopes in less than ideal conditions (windpack, crust, etc). Ski crampons will solve this but may be overkill in many situations (and add more $$ than the wider skins.) Once you pay a little extra for the right size, you will never look back. You may regret saving $10-20 by getting poorly fitting skins. That being said, you can always pick a lower angle skin track, or carry your skis up the steep slopes. Any skin will help greatly for traveling over low angle terrain. XC wax can work too, but takes knowledge & skill to use effectively.

100s should be fine. Personally I prefer to have a little bit of edge on the snow for slippery conditions.