First of all, what's the project? If it's something where size matters, yes, it is easier to adjust the size of needle until you get at least reasonably close to gauge. It's possible to adjust your tension to get what you need, but as a new knitter it's difficult and not fun, and the object of any hobby is to enjoy it.
However, if it's a scarf or a shawl, or a knitted toy, you may be better off not worrying about it. No one is going to know or care if your two-meter scarf is 1.7 meters long. Felted objects are also somewhat negotiable because you can control the degree of felting to adjust the finished size.
Finally, if it's an option for you, lay the pattern aside for now. Congratulations on having a yarn stash with which you can make something else nice. My go-to when someone drops yarn into my lap is always the yarn company's website, on which there are bound to be free patterns you can try. Knit something pretty and size-free with the skeins you know won't do the job, and you'll know what your natural gauge is with each kind of yarn on those needles. You may find that you don't like the way your bulky yarn looks when you use 8s on it. That's OK--save it for something else, because you'll acquire more needles somewhere along the line. Some yarn is too nice to get mad at.