If you have the Quick Service Dining Plan, you get two quick service meal credits and one snack credit per day. The amount of credits you're provided is actually based on the number of nights you'll be staying at your Disney resort. For instance, if you're staying at a Disney resort for six nights, you get 12 QS credits and 6 snack credits. You are allowed to use these from the moment you check in until midnight on the day you check out. You don't need to use one each day - you can use your credits in any combination you wish (three QS credits one day, none the next day, one QS credit the day after that... etc.).
As such, if you skip a day of credits for your Universal days, you just have extra credits for your other Disney days.
Unfortunately, you are not able to combine credits to use them in a table service restaurant. If you'd like a plan where you can use them in a table service location, you'll need to get the
Disney Dining Plan (1 TS, 1 QS, and 1 Snack per day) or the Deluxe Disney Dining Plan (3 TS/QS and 2 snacks per day). You are also welcome to pay for any extra meals out of pocket.
I would recommend taking a look at menus to see if a dining plan is the best option for you. The quick service dining plan doesn't tend to have much "savings" (especially if you're not a fan of dessert), and the Disney Dining Plan may have some savings if you have a lot of expensive buffet or character meals planned. I used the DxDDP once and did enjoy it, but we now go to just as many restaurants but pay OOP. Some folks just like to have the dining plan to feel like everything is "paid for" and not have to worry about it for the vacation, which may be a good reason, too.
Happy planning!