The solution is simple. Your motorcycle's owner's manual will stipulate the tire requirements. Follow them .
Likewise, tire manufacturers will publish lists of approved tires for various motorcycle models. These can normally be found online. Recommendations from reputable tire manufacturers are in line with your owner's manual. They're simply an easier way to get the same information.
If you deviate from the manufacturer’s guidelines, there is a host of things that could potentially go wrong. And since most riders stick to the rules, it’s entirely possible that no one knows how an alternative tire will affect your particular bike.
Of course, it’s your motorcycle and you can do with it as you please. Just know that deviating from suggested tire specs is like stepping into the unknown.
It’s also a bad idea to mate tires. As in, putting a radial Pirelli on the front and a bias-ply Bridgestone on the rear. If you want to know why this is a bad idea, put a different shoe on each foot and go for a nice long walk. Just like humans, motorcycles will become misaligned and unwieldy when they're on unequal footing.
So, buy the matching front and rear tire every time you shop. Also, be sure to replace both tires at the same time. Running a worn tire with a new one will have the same effect as running two different treads.
An exception : there are a handful of models that recommend mating certain tires. If this is the case, you can - and should - follow the OEM's recommendation.
When it comes to load and speed capacity, stepping up isn't going to do any harm. However, buying a tire with a lower capacity than recommended is always a bad idea. For more information on this, see the relevant sections to follow.
One last note: if your owner’s manual is “lost” (AKA, thrown away in a flurry of self-assurance), you can call our customer service team to find your motorcycle's tire specs. We tend to keep track of that kind of thing.