iMac Fusion 1Tb only reads 513gb

Maybe or maybe not, Fusion Drives are finicky and so it's hard to know for sure. That my friend is exactly why you need a good backup to Restore from before attempting to do anything.

If you make a mistake and it splits, see > How to fix a split Fusion Drive - Apple Support

If you do not have a good backup, then you need to make one or two before doing anything else.

(Time Machine and/or a Clone using CCC or SuperDuper on one or more external hard drives)

Then you need to use Disk Utility in macOS Recovery to try and add the all of the disk space back to the first partition.

see > About macOS Recovery - Apple Support

If not, you will need to Erase and Reformat the Drive back to one partition, then Reinstall macOS and Restore for your Backup.

see > How to reinstall macOS - Apple Support

If you just reformat it, yes.

In order to maintain a Fusion Drive, you need to format it in a special manner.

Try Everything You Need to Create Your Own Fusion Drive | Other World Computing Blog