Why shouldn’t even people in purgatory pray for us? Actually, I don’t see why we shouldn’t pray to our parents, for example (though I would think if we want someone to pray for us, a saint’s prayer’s would generally be most effective.)
I think Pope John Paul and Mother Teresa were the great saints of the last century, and that praying to this pope could be a good idea.
Now that I think of it, for someone to be declared a saint, two miracles have to be perfomed through their intercession. That means that people prayed for each of these people for help before they were officially recognized as saints, and these saints did hear them. I expect that this is precisely what will happen in Pope John Paul’s case.