I work in a small/medium startup with 9 people total. I have been recently asked by a Company's client, asking me to do a small project to him, outside the company (and Android app, and my company does mobile development). He specifically asked it to be outside the company, because he wants it to be cheaper, and even asked for my personal e-mail, so he could send me some requirements. I gave him my "personal company's" e-mail, becauss I don't like mixing personal and professional contacts.
I would have no problem at all to do some work outside my company, to earn some extra money, but since I knew this client through my company, I feel like it would be somewhat sketchy working "secretly" for him.
My contract does not say anything regarding working to other people, be it company's client or not, so I guess it would be legal.