Pears usually don't work as well in smoothies. Apples don't either. Try to stick with more "fleshy" fruit like the nectarine you mention as opposed to crisper fruits.
In addition, I usually only put 1-2 fruits in my smoothies. 3 or more and the flavor can get a little complicated (unless it's, say, blackberries, raspberries, strawberries, which all sort of complement each other well.
That said, smoothies are incredibly 'forgiving' so you can pretty much put whatever you want. Just throw some stuff in (maybe a little bit at a time) and see what happens! Most of the time it will at least be palatable.