Not sure what's Duo's policy in this course. In formal Hebrew אנו is completely sinoymous with אנחנו (and is probably considered "higher" language). In spoken Hebrew nobody has been saying אנו for decades.
It's definitely more "appropriate" to say שבעים. In this sense מלאים is definitely colloquial, but a very common colloquial. I'm guessing that people say מלאים a bit more often, but שבעים is still used a lot, too.