Assigning an SLA policy

As I understand it, SLA policies are only configurable based on the ticket's priority level (Urgent, High, Medium, Low) and not on any other parameter like the agent, group assignment, ticket type, etc.

This can be done under Admin > General Settings > SLA Policies:

The above example is the default rule that you have which cannot be turned off, but if you add your own custom SLA policies it will override this default rule.After you've got your preferred policies configured how you like, just make sure the SLA policies you want to use are all enabled.

When you create a new ticket, you would need to make sure the ticket has the correct priority level before you process it further to ensure that the correct SLA timer is being used for it. You shouldn't have to touch the SLA Policy itself, just the priority level and everything else is automatic.

Hope this helps!

We require SLAs that are different per support plan: Platinum, Gold, Basic, Free. Can you make it so that we can create custom SLAs that work against Custom Ticket Fields?

I am not on the top level plans, but I understand this should be possible with Estate or Forest. So have never worked with it, but the documentation says: 

"If you're on Estate or Forest, you can also create multiple SLA policies in Freshdesk for different customer tiers, products, agent groups, and so on. For example, you could have an 'Express Support' SLA policy for high-value clients".