Times shouldn't be too short if you are exposing under the enlarger rather than a plain light bulb. Stop down the lens as you would for an enlargement - no, it won't affect sharpness or anything but it will lengthen your exposure times, with the bonus that you are free to go all the way down to the smallest available opening on your lens without worrying about diffraction affecting sharpness. If times still aren't long enough you can add some neutral density filtration (get some cheap color printing filters and add equal amounts of all three colors. Note that this MIGHT affect your contrast a little anyway since they aren't pure or perfect but probably not enough to matter.

Focus doesn't matter a bit whether you do or don't.