Why not use them both?
Aleksandra KaterinaKaterina Aleksandra
Sounds pretty good to me; if you choose to go with
Aleksandra Katerina, you can still use whichever nickname you please. I don't know what this is for, a story or a baby. If it's a story, you can give it a background, like "her name was
Aleksandra, but her grandma always called her
Katia, because etc, etc" - probably some family reference. You can have her go by both names, depending on the people she's interacting with. And if it's for a baby, you can call her whatever you want anyway. She can be formally known as
Aleksandra, while she can be
Kitty or
Katia for friends and family.