Yes, the beauty of using ACH pads is that they're easy to obtain and inexpensive (it's probably supply sergeants who are posting their "surplus" on eBay at $12 for a set of 7; retail, a set of 7 runs $25-30), and they're held in with velcro. I use peel-and-stick velcro hooks on the inside of my helmet and then attach the ACH pads to the velcro.
The ACH pads also do a good job of containing the wiring on the inside of my helmet since they cover a lot of space, but don't add a lot of bulk and they're comfortable.
Your helmet should not be too tight, but it also should not move a round a lot, which is what will happen if you don' t have enough padding on the inside. I've seen a good number of helmets use either craft foam or the egg crate foam, but I like the ACH pads for when my helmet gets a little ripe, you just take them out and throw them in the washing machine or replace them, and that's not something you can easily do if you hot glue foam to the inside.
As far as other stuff, I don't have speakers in my helmet, I had to take my fans out to put the battery pack for my lights in, and I wear an over-the-ear microphone for my voice amp