Hmmmm, my s5 is fine and I'm not using any 3rd party sms app yet. I just checked and it's not greyed out, I'm able to increase it to 1000. I just got mine the other day so I'm not sure if there's an additional setting or not.
Make sure the stock message app is on as default so you can see the additional settings (when it's not set to default you can still see the other options but it's greyed out and won't allow you to select). Then within the app itself go to settings-->Delete old messages and from there you should see the option to change the text message limit