No, they won't be Chelsea, because she was unique. You miss her. What they will be, is completely unique and special individuals who you'll come to know and love for their own qualities, just as you did with Chelsea.
She was a grand old lady, and she obviously had a very close bond with you. Luna and Cleo will bond with you too, but in different ways.
My little old Codger man Paddy was 20 (anna bit!) when we lost him last year. He was very special to us - a proper little character who needed a lot of TLC and nursing in his last two years. We got Mac 4 months after, for much the same reason. He's not Paddy - he's got a very different personality, but he is a little cracker, and at almost a year old, he's grown into our lives to take his own place in our hearts.
Cleo may be timid, but I bet she still wants to belong somewhere, to feel secure and safe, every bit as much as Luna.
The good thing is that you're laughing. Chelsea isn't forgotten or "replaced". She's just left a legacy behind her for a different kind of happiness to blossom in her wake.