If you have a fairly modern revolver that is in good condition I certainly wouldn't worry about harming it with factory ammunition from reputable companies. Especially if you plan to shoot .38 Special loads in a .357 Magnum. Some posters have mentioned handloads, but I get the impression you won't be using them, if so that's a whole different story.
The "grains" we talk about are referring to bullet weight. Powder is measured in grains too, but I think you are being confused by our references to 125 Grn and 158 Grn. We are talking about how heavy the bullet is. That will clearly be marked on the boxes of factory ammo. I would not be concerned about this as far as safety in factory ammo. It is an important consideration when deciding on which load is most effective for hunting or self defense, and there is a lot of information out there about that, and of course we would be glad to help you figure it out.
At any rate, don't worry too much about harming your gun with good factory loads. You could probably wear it out with thousands of rounds fired, but I doubt you're gonna break it.
All the best.
Jim