First of all welcome to BoomerShooter, and welcome to reloading. Both are habit forming!
Yes, you can use magnum pistol primers in non-magnum calibers. You have to back off published loads though, and work back up, always looking for signs of over perssure. The first sign of over pressure is a flattened primer. The primer shoulders should remain rounded, and not flattened. Compare an unfired round to one that is fired. They should look the same.
You didn't say what the magnum pistol primers were. Small pistol primers, or large pistol primers?
.45ACP uses large pistol primers.
.40, and 9mm use the small pistol primers.
As far as the .224, 79 grain, most would suggest a 1-8 twist rifle for best accuracy, based on the weight of the bullet, but with the advent of some of the new rounds coming in the market that are longer and sleeker(higher BC) the old rules don't always apply.
Load up a few and shoot them in your rifle, and see how they work for you. If you like it, go for it!