We were given a kitten on Cinco de Mayo, and told it was male. We named him "Cinco". Then the vet told us that he is a she! So we figured we might need to call her "Cinca". As in Roberto -> Roberta. It doesn't matter that it wouldn't mean "five" any more. Would love to hear your thoughts!
If you are asking whether there are masculine names that end in "a" and feminine names that end in "o", then there are. For example:
Borja and Bautista are masculine names ending in "a"
Consuelo and Amparo are feminine names ending in "o".
So calling a female cat "Cinco" would not violate any rule. No rule says that nouns or names ending in "o" must be masculine and nouns or names ending in "a" must be feminine. That is just something that happens in the vast majority of the cases.
"Cinca" is, by the way, the name of a river in Spain.
Apart from that, the name of your cat is of course your choice. "Cinco" will sound like a number, "Cinca" not necessarily.
Spanish names are usually O for boys and A for girls. Juan/Juana.
There are a few exceptions as posted by wimi above.
However, numbers in Spanish are invariable so cinca is not the "feminine of cinco".
Uno, dos, tres, cuatro, cinco, seis= Invariable.
Cinca is merely attempting to make feminine something that is invariable.
You can name your cat whatever you want.
The connection to Cinco de Mayo will be lost if you call her Cinca.