A seven string guitar is not needed. There are bariton guitars that are specifically made for lower tunings.
Detuning a normal six string guitar is also very common. Since detuning usually means putting thicker strings on the guitar, one thing to look out for is the nut. The slots might need to be widened to accommodate the thicker strings. Another issue might be intonation. The longer scale on a bariton guitar is to allow for intonation with lower tunings. For normal six string guitars, a Fender type scale of 25.5" is preferable over a Gibson which is 24.75". That said, I have seven string guitars with 24.75" scale where I don't have an issue with intonation.
The tuning machines might not be made for thicker strings either. One can use a small file to increase the size of the holes, or peel away the smaller wire wound around the string in order to just let the core of the string pass through.
So there's nothing stopping you from experimenting with this. The death metal sound of the early 90s is partly due to detuning six string guitars while not changing string gauge, making the strings very loose. If it works, it works...