Q: Should I use boxing gloves for the heavy bag workout or do I need bag gloves?
A: It is up to you. There are some pluses and minuses to both. There is no problem in using your 12 ounce boxing gloves on the bag. But you can save the wear and tear on your more expensive boxing gloves, by using the bag gloves. Heavy bag gloves are leather or vinyl style 'mittens' that contain a decent amount of padding on the knuckles while also providing enough wrist support that errant injuries may not occur.
Do not use MMA gloves on the heavy bag unless you want to replace them soon. You'll end up wearing out the gloves quickly and risking hurting your fingers. MMA gloves are not a good choice. I recommend Twins bag gloves, along with the Muay Thai heavy bag gloves. I have had experience with all of these, and they will all work great for bag work.
Bag gloves can also be employed in other workouts in the gym. They can be used to hit the Thai pad or focus mitts. What they cannot be used for is sparring since they are far too light to be used safely.