Sorry for nit-picking^^ (and perhaps irritating you)

Ludwig van Beethoven war Komponist.
Marie und Pierre Curie waren Physiker.
Marie Curie war auch Chemikerin.
Robert Schumann war Pianist, und Clara Schumann war Pianistin. 

Sind Clara und Robert Schumann Sänger? (Considering that someone hears their names for the first time and doesn't know they're dead, this question might be reasonable.) - Nein, sie sind keine Sänger. (Ergo this reply would be correct.) Sie waren beide Komponisten und Pianisten. (But when correcting the other person and feeding them new information the tense changes.)
Was ist Albert Einstein von Beruf? (Again, in case you don't know he's a historical person, that question seem legit.) - Er war Physiker. (But the reply should emphasise on the verb being past tense.)

These people are all dead already. It is strange to use present tense with the deceased. Use past tense instead.
I understand that at this point you probably hadn't studied past tense yet. In that case it's better to use present-day people for your example sentences.

頑張って下さい