Just to add - Theo is not 'unusual'. Refer to this data for a full breakdown of popularity (table 6).
1,407 babies were named Theo in 2011, placing it as the 50th most popular name. Other names of similar popularity include Callum, Sebastian, Connor, Jamie, Toby, Harvey, Michael, Nathan, and Harley. Theodore was the 124th most popular name, given to 505 baby boys. This makes it about as popular as Nathaniel, Dominic, Patrick, Olly, Zac, Kaiden, Kay, and Morgan. While there will be some regional variation in popularity, I think you will be hard-pressed to describe any of those names as unusual.
I don't think popularity should be the deciding factor when selecting a name but it seems to me as though you are seeking confirmation of all the reasons not to use Theo, which is why I am pointing out this to you.
Arlo was given to only 158 babies, ranking as the 281st most popular boys name. Names that were used approximately as frequently as Arlo include Lincoln, Brooklyn, Isa, Rafael, Micah, Shane, Bentley, Cooper, Marcel, Rupert, Troy, Fabian, Robin, Timothy, Josiah, Rylan, Cohen, Zayn, Jeremiah, Solomon, Hector, Curtis and Gethin. I'm sure you'll agree that those names are far less commonplace.