Based on one (or more?) case properties, your goal is to dynamically decide which SLA rule to associate an assignment -- is that what you are asking?
For one, an SLA rule can be circumstanced. It is possible to circumstance using multiple properties, what is called "Multi-Variate Circumstancing". Very easy to circumstance based on a single property.
An SLA can also be configured by property name. Note how the OverallSLA flow uses .pySLAName. You could use a Decision Table to set that property value prior to the assignment.