I think that in general for a beginner/introductory *ix user that having the fewest partitions can work until more is learned about the nature of the system. However, you can't simply have one paritition and in your case, sir, for multiple reasons.
The first and more publicly practical reason for this is that most all Linux Systems require a swap partition (generally it should between 1-2 * your RAM) also require a separate system, boot or home partitions and in the case of UEFI booting linux systems an EFI partition (just 500MB).
The second reason, specifically applicable to your situation, is that taking 6x 300GB drives and making them a raid 6 is not, to say the least, the optimal arraingment. Although, new tech, hails Raid 6 as a universally better system the striping algorithm is more required and the space required to store information (compared to RAID 5) is larger in size.
Not to mention that RAID 6 requires additional piece of hardware. Which should be used, in my respectful opinion, in your case to purchase larger disks to avoid disk failure, downtime, disaster recovery or added tech help costs. I understand that some, many maybe, will disagree with me and I want to re-affirm that when disks become larger in size over the next two years ( as they've dropped in price over the last few ) RAID6, for larger arrays and large income corporations will be an obvious choice. However in this case, I do not suggest the use of RAID 6.
To the second (non RAID based) issue, creating one giant partition when mirrored may work for you however, if you want to maximize efficiency use larger drives and multiple partitions. That way if you have a double-disk failure you will have no downtime on certain mountpoints or little depending on the /dev+/dir.
At least make your /sys (system, kernel, etc) run seperately so that if for any reason your kernel decides not to boot you can simply use a recovery kernel, remote boot or PXE, disk boot, etc your kernel and have company-wide access to your information while the d-recovery process takes places.
Your company may not care about these accents so long as the system works but I'm trying to explain reasons why people do things. I would also love to hear more from others, for and against this argument and to raise other points. If you don't agree, let me know why. Poster-- I will PM you some links as well.
One Love for our Linux Community
PS Especially on network servers or systems available to the general public, even if via credential, it would be truly best to seperate your partitions. Others can input more here, I need more coffee.