I had the same piercing as your having a 6 Gauge, and others on this forum will tell you the same as I am about to, you will find it addictive once you get started as the previous poster wrote 0 Gauge is usually the line they say not to go beyond as beyond this the chance of the hole returning to normal is more difficult however not impossible.
I have set my target size at 0 or 00 but I still need to persuade my partner !
You said you have done your research but a couple things to ensure happen when you get your piercing...
1.They use a large gauge needle,
2.The initial piece of jewellery is either Titanium or Medical Grade Surgical Steel,
3.Everything is sterilized in an Autoclave.
4.You have been given proper aftercare advice(However this can be got from this forum)
As I said you probably know all this but just be sure.
Regards
Dr Rob