Reported again 14 April 2018.

"You and I" = "we" which requires "somos". Also, the verb to use here is " ser", not "estar".

A form of ser is used here because the predicate object (the thing to the right of the verb) is a noun. If you have a noun object, you use ser. Estar only takes adjectives (unless it's part of a verbal construction, of course).

Somos is the nosotros conjugation of ser. It means "we are" and is used when you're talking about a group that includes the speaker ("we").

Son is the ellos/ellas and ustedes form of ser, so it translates as "they are" or "you are". It's used when you're talking to a group ("you", plural), or about a group that doesn't include the speaker or the listener ("they").

We need to learn all the possible translations. And Duolingo needs to provide them. I am not talking about the many ways stuff can be said in English here.

There is no reason why Duolingo shoukd let us know whether the answer should be amigo or amiga. Either should always work. And if either does not then both aren't in the database as correct answers. And whichever isn't needs to be reported so it can be included.

Yes. Call it what you will, but in Romance languages, a group is feminine only if it is exclusively comprised of women. If a single man is included, it is masculine. At minimum, this is the case for Spanish and French, but this may also be out of date as it was a rule of thumb I learned in middle school French over a decade ago.

You have to see who that resulting group consists of. Since the speaker is including themself in the group they're talking about, the resulting group will be a "we", so you have to use somos here. Basically, "You and I, we are friends."

Which exact translation of "you" you use here doesn't matter. It'll always be somos.

I just successfully used, "Usted y yo somos amigos.". 3-18-19

Somos is the nosotros form, used when "we" are something. Son is the ellos/ellas and ustedes form, used when "they" or "you all" are something.