Hi Steve,
Thank you for reaching out.
Yes, you can absolutely use multiple templates on your website — it shouldn’t cause any issues.
Regarding the 500 error you’re experiencing, this typically indicates an internal server issue. To investigate further, I’ll need to take a look at your site.
Could you please provide temporary administrator access to your Joomla backend? You can safely share the credentials in the hidden content section. Also, I recommend taking a full backup of your site before any changes are made.
Once I have access, I’ll begin troubleshooting and identify the root cause of the error.
Looking forward to your response.
Best regards,