Q:

Repartitioning after install - a bad idea?

My dual-boot system crashed again - the Windows part, Ubuntu still worked fine - so I decided to erase the disk and install only Ubuntu Studio. First, I thought my hard disk was failing, but that can hardly be the case, if Ubuntu was still running fine? That is not the question, I might ask about that in another question.
As is evident from the output of Fdisk below, I only have Linux on my 1 TB disk (SDA) and in two partitions, root(ext4) and linux-swap. The SDB is an SSD, with Windows 10, cloned from the HDD on SDA. So in essence I have a dual-boot Windows-Ubuntu, but with each system on it's own disk.
I want to dedicate at least half of the disk space, 500 GB, to a storage partition, for both systems, but also for backing up old documents.
I know I can create an NTFS partition, but is it a bad idea to rearrange partitions, after I have installed Ubuntu? Is it better to do it before install, that is, choose Something else, delete all partitions and make new ones, before installing.
You may wonder what's the use of asking about this, if I have already installed Ubuntu. The answer is, I will probably reinstall anyway when Ubuntu Studio 16.04 is realeased late April. I can manage with cross-platform storage through other means, if beforehand partitioning is a wiser option.
sudo fdisk -l
Disk /dev/sda: 232,9 GiB, 250059350016 bytes, 488397168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x8f922bbe
Device     Boot  Start       End   Sectors   Size Id Type
/dev/sda1  *      2048    206847    204800   100M  7 HPFS/NTFS/exFAT
/dev/sda2       206848 488397167 488190320 232,8G  7 HPFS/NTFS/exFAT
Disk /dev/sdb: 931,5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x1f8d01b4
Device     Boot      Start        End    Sectors   Size Id Type
/dev/sdb1             2048 1937324031 1937321984 923,8G 83 Linux
/dev/sdb2       1937326078 1953523711   16197634   7,7G  5 Extended
/dev/sdb5       1937326080 1953523711   16197632   7,7G 82 Linux swap / Solaris
Partition 2 does not start on physical sector boundary.

A:

but is it a bad idea to rearrange partitions

No. You can rearrange partitions whenever you want. 
Resizing though can take a lot of time when a partition is full. All your data needs to be moved to a place on the disk that is free to move it to create the partitions you want. That is easy when you have empty disks, more difficult if the disks are used more then 80%. 
As noted in comments: a power outage is the worst that can happen during a re-partitioning. And a resize will take longer the more filled a disk is so more chance for a power outage. Yes, it is a bit more risky than partitioning an empty disk if you can not trust your electricity provider.

but also for backing up old documents.

Using an internal disk to store copies of documents is just that: storing a copy of documents. It is NOT a backup. A backup should be made such that the system the originals are on can not affect the backup. Preferably that backup should also be stored outside your direct environment (ie. not inside your house). 

