Yes! :D It is grammatically flawless as long as you use it for proper occasions. It is a casual form, often conversational expression so lacking formality. (Often used btw friends)
I mean it is possible to use this shortened version when concerned noun words end with a vowel such as 바다(=sea), 주부(housewife), 판사(judge), 엄마(mum), 아빠(daddy)
ex) 나는 가수이다 = 나는 가수다 (casual, shortened)
나는 변호사이다 = 나는 변호사다 (casual, shortened)
우리는 살인자이다= 우리는 살인자다 (casual, shortened)
살인자 = murderer
In a conversational situation, many Koreans do not strictly distinguish between singular and plural. If you are sitting for a test, you may need to be a little more strict though.
살인자들 does not end with a vowel so you can not shorten.
우리는 살인자들다 (X)
가수 = singer
변호사 = lawyer
그것은 꿈다 (x)
since word 꿈 does not end with a vowel but a consonant (ㅁ)
꿈 = ㄲ + ㅜ + ㅁ
since word 산 does not end with a vowel but a consonant (ㄴ)
산 = ㅅ + ㅏ + ㄴ
아니면, ( = OR ELSE ) is used to introduce the 2nd of two alternatives. In logics, it is closer to the definition of XOR.
또는 ( = OR ... OR EVEN ) connects two or more alernatives.
In general usage: when there are only 2 options to consider, the 2 conjunctions are virtually interchangeable although 아니면 is preferred in speech.
But consider the following examples for similarities and differences:
교환 아니면 환불 = 교환 또는 환불 Exchange or refund (only one possibility, by inference)
건강 또는 부자 = Health or wealth (-> inference to possibility of both)
건강 아니면 부자 = either Health or else Wealth (-> Inference to the feasibility of only one possibility.)