We're in search of a domain name for our project, with multiple good options available within the .io zone. However, our American partners insist on having a .com domain, which unfortunately is already occupied, forcing us to opt for a lengthier name.

Do you think it's necessary to go for a .com domain, based on your past experiences?
Also, could we look into alternatives such as .do or .co and see if they'd be a good fit?

Usually, .com domains are more reputable, with some potential clients being hesitant to click on links from obscure domains - this is particularly true if your project is still in its nascent stage.
However, it's advisable to purchase all possible domain zones in anticipation of future success and growth.