Both names are nice, but I would choose
Leo. To me,
Leo has a laid back, friendly vibe, whereas something about
Felix seems a little sneaky and untrustworthy to me.
Here's a trick I use when posting on the boards about two names I'm deciding between-
What responses do you get more excited about? Do you smile and get most excited when someone gives you a positive
Leo response or a positive
Felix response? I ask this because more often than not I find that in my gut I'm already leaning one way, and this becomes more clear when I go to read what others have to say about my names.
Hope that helps! I really do think both names sound very nice with
Scarlett (which is also beautiful)!