Are you getting new wheels or just new tires on your current rims? 30mm is the depth of the rim (spoke hole to edge of the rim where the tire starts). If you have 700x23 tires on there now you probably can run the 25's. That number is referring to the width of the tire in mm on a certain rim. It could be slightly narrower or wider on your particular rim depending on the actual rim width. How close is your current tire to rubbing on the frame? Could a slightly wider (25) fit?

How many teeth are on your front chain wheel and your rear cog? More teeth in the front will make for a bigger gear (slower pedaling for the same speed). More teeth in the back make for a smaller gear (faster pedaling for the same speed).

I have no idea about skidding because I've never done it and have no desire to do it. I do know that certain ratios cause you to skid at the same spot on the tire which should be avoided.