It's a bit unnatural without 了. I think you can also say 看完电影以后
Actually, stop thinking that 了 is past tense. In most cases such as this it marks an action completed, even if that action is completed in the future.
Why is 电影以后我们去喝啤酒吧 wrong
I think it just seems more natural to Chinese speakers to add the verb, but I'd like to hear from a native speaker.
There are two implied verbs in English that you need to include in Chinese. Instead, try thinking of "after watching the movie, let's go to drink beer"
"Some beers", plural (and countable), suggests "几瓶啤酒".
What's wrong with 我们喝啤酒去吧
I think it's okay, particularly in China (I think it would be unusual in Taiwan), but I defer to native speakers.