Hi Ghosty,
Syrena and Tamara are 2 separate mermaids. Syrena (the brunette) the main mermaid is played by Astrid Berges Frisbey, Tamara (the blonde) the first mermaid seen is played by Gemma Ward and her name is never mentioned in the film only in the credits and production photography. The two actresses do look similar because they wanted the mermaids to all have an other worldly look but they are two different people and characters.
Carina is not Syrena or Tamara or even a mermaid and is a totally new character.
Whislt Syrena does have traits similar to The Little Mermaid, she is not officially recognised as being HC Ansersen's mermaid but that is a cool canon if that is what you want to believe.
Ariel and Vanessa are also different characters who look a like because Ursula used Ariel's voice to change into Vanessa to trick Eric.
Hope that clarifies things for you.