Scarlet &
Eliot would be my spelling preference. Especially with twins I wouldn't personally give one a -t and the other a double -tt ending.
Scarlet Anna Elizabeth is lovely. Nice flow!
I wouldn't rearrange the boy's combo to avoid end -t & T- start after each other:
Eliot Gregory Thomas
It still leaves the combo well balanced, just a little easier to say the names properly and avoid Eliotomas running into each other.