Scarlett is my favourite. A nice strong name. Much prefer it to Ruby - I always compare them as they derive from the same colour. Ava is pretty, but popular. Tahlia I'm not crazy about. If you do go for it, I would personally avoid the Thalia spelling as it reminds me of the Indian dish Thali - http://en.wikipedia.org/wiki/Thali - which in itself is delicious, but not sure it works for a name. If you stick to Tahlia you avoid this.
Harry and Alexander are my favourites from your boys names.
FWIW: I would have gone for Alexander but am a northerner living in the south and couldn't bear everyone pronouncing DS's name differently to the way my husband and I would pronounce it. Harry is good, Max very popular here. I don't like Reuben.
Scarlett and Thalia (or Talia) are nice names but not uncommon here (there are two or three of each in my DC's school - 200 in the school) and, sorry, don't like Ava. Ruby is nice. (Odd that I like Ruby but not Reuben. Can't explain that.)
I like Ava and Harry the best - they're popular for a reason
Also, the popularity thing really depends on the area you're in - I have a 10 month old Harry and have yet to meet another baby/toddler Harry despite going to numerous baby groups, drop ins, swimming lessons etc.
Scarlett - nice Ruby - nice Tahlia or Thalia - not keen Ava - nice, but too popular for me
Alexander nn either Alex or Xander - nice, but too popular Reuben - love it Max - fine, but a bit meh Harry - really like it and don't know any little Harry's although it is quite high up in the popularity thing.