Is it against any regulation to have a commercial network in the same environment as a NIPR and SIPR network?

Your SIPR should be self contained in a secured room. No other network is allowed in that room, including NIPR. There should be no wireless access in that room, as well. Having said that, my last unit provided a civilian commercial wirless network for general use. It was justified as an MWR resource. It should never be used for official business, unless the user is using a vpn to access DOD resources, and then only if no available NIPR workstation is available. I do not believe that such a setup is covered in any regulation, and is, instead, a local command level decision.

It sure used to be!

I would guess it's a security problem, regular network access might open a security hole in secure military networks. Just a guess though.

For example what the US Navy classifies as NOFORN (not releaseable to foreign nationals) the Army may classsify as Secret . . . Further examples within each branch at differnt levels also apply.
What an air unit in the Navy would classify is different than what a submarine command would . . .

No uniform regulation covers this. It is bound from departmental governance of information: NOFORN, Confidential, secret, top-secret.... I think (Opinion / not fact)

I have a filling it is , so you might want to not have it there.

Have you asked the BAMO? There should be a BAMO or NetOps Warrant that has your answer.

Likely no, however SDN/traditional firewalls , and proper fedramp and fisma compliance on some nodes and comercial nodes is a pain. A Few cloud hosting providers do so.
However muti-layer dmz also a factor.
YouTube has a Dod and federal government compliance company out of Fort Wayne In. One internal network supports stock business website hosting.
The others various Federal government cloud hosted products.
Verizon has been Nashin at the bit for a FISMA certified. (250 +/- or less ppl have it in USA curently .. I've been training for CISSP.. I and after the 5th offer looked up diacap/fisma)
Not saying it's a means, however it's a long presentation. However some networks in dod are traditionally isolated.
However with SDN (software defined networks ) the ability to secure cloud computing of networking applications and services, with per user VPN could help the warfighter. Training and or other items can be VPN and virtually available to military units globally. Base is hit with a tornado, then training or operations is affected. I'd the item is cloud , then multiple secure instances can be run globaly. Congress is worried over cryptography... however perfect forward secrecy is becoming increasingly posible.
So telling VPN or who were or transparent traffic that doesn't leek it's destinations, what's encypted, ie disguised as plain traffic.

While some networks won't ever be online
It may become posible to have military grade private clouds.

There are exceptions to complete separation. Like another poster, I won't go into it, but suffice it to say responses that need to be worked both high and low concurrently, things are set up so the people responsible for a given thing can work the whole picture.

I'm not sure. However, it's usually in a policy letter from a J6/G6, in my experience It's definitely looked down upon, especially if there's Wireless involved.