That's very unncommon and sounds awkward to some, better not to use it. The adjective should agree with noun in its grammatical case..

You may say it, you might hear Poles say it, it is not correct, unless before a noun.

With małą the sentence: Tamta ryba jest małą rybą

Well, first you can check what immery usually writes to questions about this/that: here

So we usually accept both 'this' and 'that' for 'ta' (at least we try), but if it's 'tamta' in Polish, it really needs to be 'that' in English.

You change the keyboard to a Polish one, as it has everything you need for English + of course everything you need for Polish ;) And then it's "right alt + l" or "left ctrl + left alt + l".

Yes. Adjectives match nouns. If there is no noun after być the adjective is in nominative.