I definitely will not fault people for putting forth this Petros/Petra argument because it is a very common argument that people make but there are a number of problems with it, and if I were to summarize those problems they would go like this:

1) First, the very layout of Matthew 16 is a three-fold blessing, and each part of that blessing is itself in three parts, a blessing, and explanation of the blessing, and a further explanation of the blessing. It's a threefold blessing of Peter and it goes like this: 

1) Blessed are you Simon son of Jonah, 

1a) for flesh and blood has not revealed this to you

1b) but my father was in heaven.

2) I tell you, you are rock

2a) and Upon This Rock I will build my church

2b) and the Gates of Hell will not Prevail against it

3) I give you the keys to the kingdom

3a) that whatever you bind on Earth is bound in heaven

3b) that whatever you loose on Earth is loosed in heaven.

That alone tells you all you need to know given the structure of this sentence and where it is situated within a three-fold blessing.

2) Of course we can go deeper than this because we know that Jesus spoke Aramaic and the word in Aramaic that he used for Peter's name is Kepha, transliterated as Cephas in Greek (well, ripped from Aramaic into Greek and then Greek into English, but you get the gist!)

The Aramaic word for a small stone is EVNA, not KEPHA. Had Jesus intended for Peter to be known as the little Stone, we would be speaking of Simon Evan, not Simon Peter.

3) as you yourself pointed out Petra is feminine, Petros is masculine. It is fine to use feminine words when speaking of men if there is no other way for that word to me made masculine, but using a word metaphorically And giving somebody a brand new name are two entirely different things. Calling him "petra" would be calling him "Mrs. Rock," so Jesus is just changing the gender of the word to match Peter's gender.

4) Multiple, MULTIPLE Protestant scholars Disagree fundamentally with what you've said and agree with what I've written, namely: 

-There is no distinction in meaning between "petros" and "petra."

-Two different Greek words are used because you can't use a feminine noun for a man's name.

-"This rock" refers to Peter.

-The identity of the rock ("petra") is affirmed by the Aramaic that Jesus was speaking.

...So this isn't just a Catholic position. https://phatcatholic.blogspot.com/2006/09/protestant-scholars-on-mt-1616-19.html?m=1&fbclid=IwAR3vP4x1_VMC74-QrwDKxRD59ir0_PxvxYk1yVeBCGxEsshI_zvHo-vD3oU 

5) NONE of this necessarily actually denies that Jesus is also the rock, OR that the statment of faith is ALSO also the rock. In fact, the Catechism in paragraph 424 states exactly this: "Moved by the grace of the Holy Spirit and drawn by the Father, we believe in Jesus and confess: ‘You are the Christ, the Son of the living God.’3 On the rock of this faith confessed by St. Peter, Christ built his Church."

BUT that doesn't preclude the other, more OBVIOUS meaning state above from being true (and the church of coures also understands that the be the principle interpretation, CF: http://www.catholic-catechism.com/ccc_153.424.440.442.552.553.586.849.881.1444.htm )

Hopefully that's helpful!

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