The short answer to your question is, yes resizing your Ubuntu partition from your live medium should not cause any issues with stability.
IMPORTANT: If you're concerned about losing data. Back up the files you care about before proceeding. Any shrinkage or movement of partitions has the potential to cause data loss (though it is typically rare and has never occured in my experience). If you possess an external hard drive or a USB that's large enough to contain a clone of your current partitioning scheme, I'd highly recommend creating a bootable CD/USB of Clonezilla, which can create an exact copy of your hard drive in your machine and restore it if something goes awry. It's saved me numerous times.
Resizing your partitions
To resize your Ubuntu partition, you will need to boot from the live CD/USB and use GParted from there. Once inside GParted on the live CD/USB, select the Ubuntu partition and shrink it to your liking.
Warning: Do NOT use GParted to resize/move the Windows NTFS partition. While GParted is capable of doing so, this can cause issues on Window's end. Back when I dual booted Windows 10 and Ubuntu, I used GParted to resize the partition Windows resided on, and it prompted Windows to insist that the filesystem needed to be checked for errors. Luckily, after running check disk, everything was fine; however, data loss is entirely possible. Once, you've changed your Ubuntu partition, reboot into Ubuntu to ensure that your files are intact and nothing is damaged.
Note: I haven't been in your exact situation, but I would imagine that unallocated space will be separating your Ubuntu partition and your Windows partition after shrinking the one Ubuntu resides in. This will be an issue if Windows cannot manipulate that unallocated data on its own (as it is behind the Windows paritiition). Try what I've described below.
Next, boot into your Windows partition and open up the Disk Management Tool. From there, you can add unallocated space to your Windows partition and extend it. If for some reason Windows cannot manipulate the unallocated space after the Ubuntu partition, then you have a couple options:
- Download a third-party tool such as MiniTool Partition Wizard, EaseUs Partition Manager, and others. Personally, I have not used nor can I vouch for either of these tools. LifeWire actually has an article called 10 Free Disk Partition Software Tools that was published recently (3/06/2018) where you can find a few more utilities with brief descriptions.
- Use GParted. GParted was actually mentioned in the LifeWire article I linked to. Despite my previous warning to not use GParted to resize/move your Windows partition, I trust GParted more than some of the other utilities out there.