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

In the Infantry your unit controls what you do to Govt issue. I warned some of my folks not to go all HIGH SPEED when they went ahead and painted their M-4's and on the sly added gear queer crap. When we got home most of my guys just needed to clean their weapons and turn them in, the high speed low drag self appointed commandos spent many extra hours returning their weapons to original condition and it was not fun for them. I watched grown men purchase thousands of dollars of crap to trick out their weapons only to stay behind the wire the whole deployment. I guess they looked good in their selfies though.
I understand you excitement and curious mode but it would be best to just keep a low profile about certain things till you get to your unit and see what the situation is like, many units are different. Believe me when I say that after well over 35 years I have seen almost all of it and this is the best advice that will cause you the least blow back.

Do not be the private that gets out of basic and buys all high speed equipment. The E4s, E5s and 6s will notice and clown you for it lol you want to remain low profile and just do what you gotta do.

The answer is yes. The only limitations on this will be at the discretion of your CO, 1SG, and the armorer. Strangely enough, if the armorer, regardless of their rank, doesn't want to deal with your stupid stuff, then you can't do anything about it; it is after all their property after they sign their hand receipt. As far as customizing weapons goes, when I was an armorer, I allowed soldiers to switch out anything besides the upper receiver, lower receiver, and the inside guts of the weapon. Pistol grips, hand guards, stocks, optics, if you wanted to invest in making that M4 something crazy, then you could go for it.
Usually, no one cares what you do with your kit. If there is an SOP set by the unit, then so be it.