Is it ok to trim down a tube patch? I got a 700x18-23 inner tube that just got punctured. The small round patch on my patch kit is a bit wider than the inner tube itself. Is it ok to cut around the patch so it fits? 

An added question: when buying an inner tube,  should i buy the smaller 700 x 18-25c, or the 700 x 25-32c for my 25c tires?
Don't cut it. The patch is thinner at the edges and cutting leaves a thick sharp edge where it can tear. Roll the tube sideways and you can glue first one and then other edge. There are smaller patches that are made for narrow road tubes.