It's really about how you and your team are different in the end. Yes, there is always that risk...it is just how things work in the startup world. No one is signing NDAs and your enhanced feature has likely been thought of by others. It is better to try and get your product / feature out into the world than hold it for fear of being poached by the big guys....they likely have bigger problems and opportunities to worry about. Take reasonable precautions and go for it.

There are many resources to find mobile developers now. You will have to balance your investment level with reliability and quality. sites like Odesk, eLance, Freelancer and others are great resources to post a project and await many bids from developers all over the world...it will be very difficult to really gauge the quality of work...especially if you are not technical enough to review their work. Ensure you indicate very clear requirements and samples. There are often general differences between country of origin. I strongly recommend you work with a fixed bid and not hourly. Ultimately, it is best to find someone local through Meetup.com events, local colleges/universities and referrals from others. Sometimes, you can get parts of what you need done through low cost resources. Good Luck!

There has been a great deal of activity in this space especially over the last 1-2 years. There are various mHealth and digital health sites, groups, conferences and resources. I recommend you check out Aging2.com as one place to start. I have been working in this space for several years and can share what I have learned.

I recommend http://www.startuply.com and can share other resources and strategies.