More or less fine with other verbs like "kommen", but still can't really get the difference in the feeling particularly with "sein" because it's quite special... Let's say
- Es gibt eine Katze hier.
- Es ist eine Katze hier.
- Eine Katze ist hier.
- Hier ist eine Katze.
Which feel differetly and which are totrally interchangeable?
All of them are fine and mean the same thing. The differences are minute. It's more a question of personal style and very slight shifts of emphasis and thus totally dependent on context. Without context I'd personally prefer "Hier ist eine Katze" as the default, but all of them are fine.
Still confused about the "es". I use the example sentence "Ihr seid alle gekommen" which is essentially the opposite of this one. Can I assume that the "es" is in the same functional position as the "ihr". "alle" is the opposite of "niemand" and the structure of the sentence is the same? Given that there is no one instead of you(all) an "es" should be inserted?
An example of this is seen in the beginning of Hitler's speech at the 1934 Reichsparteitag speech where Rudolf Hess introduces Hitler by saying "Es spricht der Fuehrer". Es stands for a syntactical place holder for "spricht". Odd example I know, but I saw this on the History Channel once and found this use of es very peculiar. This is a common video clip seen on many History Channel videos of Germany during WWII.
No. That would be like saying "Nobody come" in English.
You can say "Nobody came" (with the simple past) or "Nobody has come" (with the present perfect = helping verb + participle), but not "Nobody come" with just the past participle.
Similarly in German: you can have "Niemand kam" (with the simple past) or "Niemand ist gekommen" (with the present perfect = helping verb + participle), but not "Niemand gekommen" with just the past participle.
(Actually, I lied: you can say that in "telegraphese". So you might find that in a telegram or a newspaper heading or some other place where you want to save space. But that obeys different grammatical rules. So just forget I said that :D)
English tenses don't map one-to-one to German ones.
In particular, German uses the present perfect a lot more than English, especially in the spoken language (the imperfect or simple past is mostly just used with a handful of verbs or in books).
So "Es ist niemand gekommen" could be either of "Nobody came" and "Nobody has come".
And "Ich habe gestern ein Buch gelesen" and "Ich habe jeden Tag Eier gekocht" are perfectly fine in German even though "I have read a book yesterday" or "I have cooked eggs every day" do not work in English (present perfect plus time expressions doesn't really make sense there).