i'd say stay with a 3.5 oz Co2. Phantoms run better on Co2 out of the box because Co2 has the correct operating pressure, and you get a lot of shots off a small size tank compared to a tank of the same size using HPA. Shot to shot consistency is not a problem unless your using an open class and you auto-trigger often, if you not part of that category Co2 will work great. If you do choose to use a 13ci some people say it's too long, and it's technically at more risk for getting messed up if your not careful, BUT as long as when you slide you turn your gun 90 degrees either way you'll be fine. If you play a lot of woods ball, it may be at risk if your running and trip and land on it, but by that point, so is a 3.5 oz Co2 tank. It is not by any means too much weight, and NO it will not bend the trigger frame or snap it just from being on. THERE ARE NO MODIFICATIONS NEEDED TO PUT IT ON, but if you want it to run as well as Co2 does you may have to play around with springs on the phantom and output pressure on the tank.
In my OPINION, I would have to say stay with a 3.5 oz Co2 tank
1. I have never been more tired after a day of paintball. 2. 10 feet shots hurt 3. I love my Phantom VSC when using 12ies 4. I will not be using my Dangerous Power G3 for quite some time. 5. I'm covered in Mud. 6. I've never gotten so many faces staring at my marker since I first got my pinokio
I lost a few 10 round tubes, had some fun, got some people out, got my ass kicked, and loved it.
same with me today, was like my second day with my phantom ^^