1) The difference between those kids and Dick was that a) Dick wanted revenge, he wanted to kill Zucco, and Bruce wanted to make sure Dick didn't grow up to be an awful person; hence all the different stories where we see that, without Bruce's guidance and love, Dick would've become a two-bit hood just like Chill or Zucco and b) Dick had no one, not even an estranged relative, to take care of him, just like Bruce; personally, I think that's more important than the two of them being the same age when they lost their parents.
2) That is true, but honestly, I think that the whole father-son bond still works if Dick is 13 when they first met. As for deepening that bond by having Dick be 8 when Bruce took him in, I'm willing to sacrifice that aspect if it means we don't get that bit of weirdness with "wait, Batman let an 8-year-old fight crime alongside him?" and "wait, so how could Tim have witnessed the Flying Graysons' deaths if that happened when Dick was 8?"