The negative prefix on the adjective does not matter, the tag question is determined solely by whether the verb is negative or positive. The correct usage would be "I am impatient, aren't I?" There are occasions for when you could use a positive-positive tag, but it's not really a question (the "up" inflection isn't there). It's used to confirm another person's statement often expressing mocking, doubt, wonder, anger, etc.
Example
Person A: I was sick yesterday and spent the entire day in bed.
Person B: You were sick, were you? Then how come I saw you jogging in the park?
Person A: My wife is going to have a baby.
Person B: She is, is she? Congratulations!
Tag questions (short answer) in declarative sentences:
A tag is formed by taking the helping verb of the main verb (make it a question to find the helping verb).
Everybody forgot to congratulate her.
Did everybody forget to congratulate her?
TAG: Everybody forgot to congratulate her, didn't they?
Main verb: forget Tense: simple past (forgot).
Also, don't forget that in a declarative sentence tag, the tag must be negative interrogative (didn't they?). In a negative sentence, the tag must be plain interrogative (They didn't forget to congratulate her, did they?).
You can practice with these:
The boys have found the dog and brought him home. [tricky, kind of]
You will be going to school this afternoon.
We wouldn't have been in town so late, normally.
Clues: the helping verbs are either in the sentence (have, will, is/are, would/should) but in the present and simple past, you have to come up with [do/does and did).
You speak Russian, don't you? You have to remember that do is the helping verb.
This is just a basic introduction, not the whole entire story.
Here is an important irregularity:
I'm being clear, aren't I? :) [regular]
VERSUS I'm being clear, am I? [in response to praise or to be sarcastic, for example]
That one does not follow the rules, you just have to learn it. First person sentence in the present tense with a tag.
Also, in a negative sentence: I'm not being clear, am I?
Best Answer
For the first example, isn't it is correct. Don't we does not make sense in this context. The sentence should be:
The second example, I would not use either option. Instead, I would use isn't it here as well:
The reasoning for this is that it coincides with the beginning of the sentence. For example, I might use any of these:
Don't and didn't are exceptions to this: