"No, they are not tall."
Translation:Ne, nejsou vysocí.
You can use any gender you want. In real use it will depend on the context. The translation above does use animate masculine and the adjective is in the nominative case.
oni is masculine animate
vysoká is either feminine singular or neuter plural (cf. with eg. Latin)
You should be able to check that yourself in the Tips and notes.
On the web click on the lightbulb icon when choosing the skill.
When you first start a lesson, the little box which says what lesson you are on has two icons. One is a key, and the other is a lightbulb. Select the lightbulb for Tips and Notes. I had to ask, also! Unfortunately, this only displays in Duolingo on the web, not in the mobile application.
Why can't use "Ne, oni nejsou vysoke?" If the thing is masculine inanimate?