It's not that you CAN'T use the line (I've seen people "cast" a 5 wgt line with no rod!)
You will find that that what you are describing is about the limit for effective use of your rod. What I mean is, a one line weight difference is OK, and in fact makes some rods slower, which some people like. A two weight difference will, as the poster above said, load your rod - but not "faster" it will simply make your rod feel a bit "sluggish" as the rod will bend way more than it was designed to. This means if you have a slow casting stroke, and are patient in letting your line straighten out in back of you as well as in front, you will be able to get the thing to work, but this will obviously limit your casting distance. I would also suggest that casting into a wind will make it a bit more difficult as well - because you won't be able to build line speed in your casting.
In short, yes, it will work, but a proper line is so much better.
The reel is a whole other matter. Fly reels are made to balance the rod. In a properly balanced outfit, with the reel on the rod and with line in the reel, you should be able to balance the outfit on one finger with that finger somewhere in the handle of the rod. If you find the balance point is somewhere in the back of the handle, it means the reel is too heavy, and the further in the back of the handle the balance point is, the more out of balance you are. A back balanced outfit can be cast OK, but over the long run of a day's fishing, it will again require more effort from the angler - because in fly fishing you are constantly casting. This, to me, is more serious than the line weight, mainly because when I go fish for a day, it means quite a few hours of casting, and an unbalanced outfit is a pain (literally) in the wrist. The whole issue of balance is called "rod swing weight" and a good article online is here.
My advice would be -
If the reel balances the rod in the middle of the handle, or close to it, it's probably OK to use.If not, like if it's way in the back of the rod, consider another reel.
Try the line, if the rod's slower action doesn't suit you, you will need a new line of the proper size.
Both these things can be had for not all that much money at many, many stores.
Why limit your fly fishing with ill fitting gear?
Hope that helps,