Universal is pretty small. One full day, including all the rides, Harry Potter and the shows. It will work very well if you show up early in the morning (when the park opens). Recently, my friends (who were staying at my house) got there when the park opened. They were able to do most of the rides before lunch. And that was a Saturday before July 4th.
For the parks: If you are there tonight, go to the main entry right now and go get tickets. They will not be validated until you actually cross the turnstills and will be good for 13 days. If you have three or four days, I would say a one-park a day ticket is perfect. Recently, I did two days, one per park with my friends and that allow us to do everything and more. (I am an annual passholder and go ALL THE TIME).
Universal is a long drive from Disneyland (at least one hour if traffic is great, more like 2 hours). If you still have time, I would go to Universal on Sunday morning as traffic is the easiest on Sunday morning. If not, I would leave Disneyland no later than 6 am (I would go for 5:30, personnally) to get to Universal around 7am, way before traffic. If you are still in Disneyland at 8am, I would not bother getting on the freeway.