The magazine issue is a non-issue, as others here have stated.
Get training. Lots of it, from different instructors if possible.
Practice. And practice with your carry load. You can use FMJ but if you are shooting JHP for self-defense, practice with whatever brand you are carrying to make sure the gun "likes" the ammo and won't crap out and stovepipe or something every 3 rounds.
Talk to a local lawyer who has experience in gun cases - preferably winning them - so you KNOW what the local gun laws are, how they are applied by the police and prosecutors. DO NOT talk to a cop about this - odds are they are wrong. Talk to a successful lawyer. And pay him, get his card. God forbid you ever need it but have it with you just in case.
Get more training. And practice some more. What's that old saying? You revert to the level of your training mastered.
CCW is a trade-off between stopping power and concealability. I have carried a .380 only because the Ruger LCP is so small and light, you can carry it in a DeSantis Nemesis in a shorts pocket and no one will know. Not exactly a huge round. The joke with the .380 is that if you shoot someone with a .380 - and they find out - they will be pissed!
OTOH, a Beretta 92 is not exactly the most concealable of sidearms. And of course it is a 9mm.
Understand, however, that NO pistol caliber is considered a man-stopper. Clint Smith jokes that he carries a pistol to be able to fight his way to his rifle which he should never have set down in the first place!
With that, bigger is better - in caliber and in mass if you are sensitive to recoil. How big has no right answer, only personal preferences. You have to decide how you will carry, what makes the most sense in your specific circumstances.
So you want to carry the largest caliber you can comfortably carry and conceal 100%. I doubt seriously that the MA cops will give you a pass for inadvertently "brandishing" the weapon, even if all that means is that your shirt got blown up by the wind showing part of the weapon on your belt. Some of my friends who are LEOs when off-duty will wear their badge right next to the holster under their shirt so if someone sees the gun, they see the badge too. But LEOs typically don't need to worry about "brandishing" charges - civilians do.
Based on your body size and shape, and the clothes you normally wear, decide on whether you're going to use an inside the waistband (IWB) or outside (OWB) holster, pocket or ankle holster, or - be careful here - off-body carry using something like a Safepacker. Off-body carry is useful in specific circumstances (driving in a car, bicycling, etc.), but carries even more risk, namely leaving the off-body case somewhere and forgetting about it.
Gun shop people are generally knowledgeable, but they are human and have preferences too. So using the criteria above and in this thread, decide what works for you, and do not be afraid to change it if it is not working as you expected.
Good luck, and congrats on the CCW. In MA of all places!!!!