I own 7 pound dumbells and use them all the time during S&H on rear delt raises, lateral raises, after I've given up on the 8s! Personally, I would have no use for 4, 6 or 9 pound dumbells. I have a pair of 1 pound wrist weights from way back when, and those can help you go from 8 to 10 pounds as well as from 3 to 4, or 5 to 6. Might be a better investment than getting the individual dumbells you mention.
But by far your best investment will be a pair of 20 pound dumbells, which you are going to need for STS and platemates. Platemates can help you on the lower weight increments, from 5 to 6 pounds, and also on the higher weight increments, from 20 to 22 and half pounds dumbells. So platemates are your best investment overall. Get two pairs of each size since you need both dumbells for certain exercises, like bicep curls.
Clare