Here's a more detailed answer to my own question based on the experience I had.

As soon as a few weeks after the spouse applies for a Family Reunification visa through the German embassy, the one living in Germany will receive a letter attached with 5-page forms in the mail from an Auslandärbehorde officer. The visa process won't go through unless you fill out the forms properly and send them back to the Officer's email provided or mail them to the Auslandärbehorde office for the specific agent your case is assigned with.

Letter Mentioning Required Documents

So, you'll be asked for:

1. Recent employment confirmation (form attached, needs to be filled by your employer to confirm that you're still employed with them)

2. Last 3 salary slips.

3. Rental contract with the indication of living area, or the attached form filled by your landlord. (Even my rental contract had the living area, they asked me to approach the landlord and fill the form anyway)

4. Bank statements for 3 last rent payments.

5. Community Home form

6. Living expenses form

What concerns this specific question, is the 2nd and 4th items. Below are German samples. Suit yourself with Yandex OCR translator, if you need an English translation.

2. Landlord Confirmation

As you can see, the Landlord needs to confirm how many people are you living with as well.

4. Community Home Form

So, you'll be explicitly asked if you're living in a community home and if so, with who. Per legal grounds, if you got the minimum living space area for you and your spouse, you should be fine. But the final decision is clearly theirs.

I personally decided not to live in a shared room, so I have no personal experience with this scenario myself.