In an Infantry division, can you customize your rifle and some of your gear?

NO, NO, NO! The unit will give you what you need and must have, and equipment has to be turned in the exact same way it was issued to you! With that being the regs- When I retired, I had over 3 sets of gear- 1 that was issued and kept for layouts, 1 for special inspections, and 1 for the field. This didn't count extra desert, jungle and artic gear. My field gear was rigged for stealth, mobility and use- but your leaders have to be willing to ignore it.

Iit is government equipment and isn’t altering some form of destruction of government property

If your unit allows it, yes. I had an aftermarket "gangster grip" with integrated led lights for a while, but normally I was fine with the stock weapon.

EDIT: If you are allowed, please don't try to be Mr. Tacticool Guy!

You can customize at a minimum. Honestly there are more important things to worry about

Some units allow it, others don't. It all depends on how your Top, and brass feel about it. To be honest it looks cool but when you have grown accustomed to it and you really need it it's probably not going to work. You'd be surprised on how many times I've seen shit that's supposed to be military grade fail and the guy have to go to iron sights. At best get a triangle and stop on your rail and stick with your ACOG and some iron sights on a 45° offset. Lasers give away your position when it's foggy or dusty. IR lasers are great target designators when they work (not all function well depending on what type of NVGs you're using.) And besides the old school guys like me that came in during the era of BDUs and iron sights are gonna fuck with you until you cry. You also have to consider weight. When you're a light infantryman (11B) you have to carry all of your shit. Trick out your rig along with all the other essential shit you need for daily operations/patrols and I promise you that you'll belay any unnecessary weight as soon as you finish your first 15 mile road march. As for your ACOG definitely get a quick detach riser. They are incredibly helpful if you wanna switch out optics to your tricked out crimson trace x-ray automatic speech enabled laser target designator that goes pew pew pew when you pull the trigger and suck starts your pickle in Joe's jack shack at the end of a long ass patrol!

Yes, here is a picture of my issued rifle.

Not recommended to do it.

It’s at your chain of commands discretion. But possibly, yes

To an extent yes. But don’t go all out at first because of three reasons. First you need to find out what your company will allow, second you need to find out what works for you cuz there is nothing worse than buying all that stuff and finding out you don’t like it, third you don’t want to show up looking all “tacti-cool” and not even knowing your actual job (you will look like a dumbass)
But honestly you don’t need all that stuff all I put on my m-4 is a forgrip that turns into a bipod (I got it from the armsroom) and a flashlight attachment and I shoot a 40 out of 40 every time I qual.

Don't be that guy.

You'll look like knob.

MSG Bo