Can I place file backups on a disk with the Time Machine backup?

if it's a TM disk that has only big sur backups, then no, the disk is a read only disk. also, it's not recommended to keep all of your different forms of backups on the same disk. if it fails, you lose both backups. you should also consider a bootable clone of your local drive as well as an offsite backup incase of theft, fire, or natural disaster. that way you'll have a backup in a worst case scenario.

Agree with previous posting all eggs in one basket is a invitation for disaster.

Suggestion >> Do I have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

you're welcome.