I've had mine for 12 years or so now and from what I can remember it really didn't hurt when I got it done. IT took all of 5 seconds for it to be done and over with. It wasn't even sore or anything that day and so on. I cleaned it with GLYoxide multiple times a day and I even changed the jewelry with in a week.
As for having it pierced with a retainer, I'm not sure what the chances are of getting a professional to do that... If for only one reason, the retainers are either acrylic or plastic... Surgical stainless steel is much easier to sterilize. Reducing the risk of infection and making the healing process faster.. If you really want to get it done though. I would say go for it. Pick a day to get it done right after you get off of work, and a day that you'll have the next, hmmm I'd say two days off. And then before you have to go back to work I would buy a retainer, soak it in glyoxide or diluted Listerine, make sure you clean the piercing sitewell, your mouth, hands and retainer and just change it yourself... Note though one you change it to the retainer I would leave that one in for a couple of weeks... You don't want to irritate a new piercing by constantly changing it each day... Once it's healed then I would start doing that..
As for me I have a job where I have to remove it for work, but like I said I've had my piercing for 12 years so I can take it out with out the fear of it healing up anytime soon...
As for gauge size don't make the mistake I did. I had mine pierced with a 12g... Which is flipping hard as fuck to find any post in that size.... The average seems to be around 18g, 16g or 14g. And the longer you have it the more relaxed it gets, I often where a 10g hoop in mine when I want to change it up a bit..
Anyways hope some of this info helps... And good luck.