For example 新メンバー instead of 新しいメンバー、。
Q1: Can they be used interchangeably or are there any rules when it is better to use a certain one.
Q2: Does this “abbreviation” work with every noun or are there again rules and it only works with certain nouns.
For example instead of 新しい自転車を買った。could you also say 新自転車を買った。？
Q3 (last one): Is this possible with every (い) adjective?
For example 古車 instead of 古い車。
可愛猫 instead of 可愛い猫。
青海 instead of 青い海。？
3 comments
I don’t know, but isn’t 新メンバー just a compound noun? 新 is a word (prefix/noun) in its own right (しん – meaning new) as well as a component of the i-adjective 新しい. Same for 古.
My guess is that it’s nothing in particular to do with i-adjectives, but just joining nouns together to form compound nouns.
I would not prepend 新 to make new nouns and instead just use the adjective form, and only use the prefix form if you’ve specifically seen that word before.
新 (しん) is its own thing. It has its own rules, usually as a component of a word rather than a separate word modifying a different word.
Some adjectives have this form, others won’t. You’ll have to study more to see.