Non-cap h1 to cap h1?

Hi folks. I am currently on F1-OPT status expiring Feb 2015 (No STEM). A staffing agency (who partners with a research institute) is ready to sponsor non-cap h1b visa for me right away and is also ready to file cap h1b in April 2015. Does this sound okay to you guys? Are there any risks involved in this? I mean filing non-cap h1b and then cap h1b after like 3 months. He is also ready to go for premium processing for non-cap h1. Is this necessary? Would I not get cap-gap work authorization extension since my h1 will be applied during my OPT? Also, I am working for another employer as a volunteer since the time my OPT started. Would this cause any problem during h1 filing as I would have no pay stubs. He will be providing me a letter saying I worked for him as a volunteer or an unpaid intern. Is a letter saying unpaid intern considered better than saying volunteer? Thanks in advance. Would really appreciate any advice that you can provide.