If you intend to be informal, by all means, use the personal pronouns you, we, they, and I. And you can toss in the occasional clichḗ and colloquialism if they get the point across with impact.

The more informal letter cut the length by half, included a personalized greeting, and used informal tone and punctuation (i.e., “We want to work with you!” and “Good luck in the new year!”).