Is a partition the way forward?

Hello, folks!

Forgive me if this has been asked before or if I use the wrong terminology, a bit of a newbie.

I have a new imac 27 with a tb fusion drive.

At the moment I am running all my files off an external hdd, and all my software is on the mac itself.

Is it possible to partition my drive, so that all my files are on the hdd part of my mac, and the software will remain on the ssd part?

Is this wise?

At the moment its super speedy, and have read that if i start filling the drive with my files the mac might show signs of slowing down.

It would be nice to keep just the software on the ssd.

Any advice ?

ps, I usually deal with photoshop and illustrator, and load speeds are acceptable from my external.