I love the name Kate too and I agree not that popular at the moment. I think it is an elegant classic name and so doesn't sound too plain with Smith. In theory having first and Last names with different amounts of syllables does sound better but I think Kate Smith is a great name and if she gets married and changes her name then it wont always be her surname anyway.

I personally am not the bigget fan of Katelyn and think that it sounds a lot more plain with Smith thatn Kate does. Also I think if you are always going to call her Kate then she will be known as Kate Smith so you have kind of just given her a name that you didnt really want just so she could end up being KAte Smith anyway iykwim? I know a few people who named there kids longer names and wish they just went with the actual name their kids are known by. Just my opinion though.

I think the name goes wonderfully with your other kids names too