1. "And I assume they are not precise either" is quite good. It's a bit informal, however. So in conversation this sounds even more natural:
And I assume they aren't precise either.
In writing, on the other hand, "moreover", "furthermore", or "also" would be appropriate. Don't begin a sentence with "also":
Furthermore/moreover, I assume that they are not precise.
I assume, furthermore/moreover, that they are not precise.
I assume also that they are not precise.
"Too" is not quite good here.
2. The rhetorical confirmation -- where you are expecting a yes or a no -- is negative for an affirmative sentence, and affirmative for a negative sentence:
I assume they aren't precise either, are they? (expect a NO! -- which means they are NOT precise.).
They are good, aren't they? (expect a YES! -- which means they ARE good.)
"I assume they are not precise either, aren't they?" is a very odd sentence.
Student or Learner