Can an entity type mix multiple entity types?

If yes, does this cause any strange behavior?

no, unless in hierarchal fashion. This is the same constraint as java to avoid the diamond problem: Also table naming wouldn’t work because of multiple scheme

That makes sense. Is there a way then to have an entity type use multiple interfaces and have a reference to one of those interfaces? In c3typs, something like this:

type TypeA {
type TypeB {
entity type E1 mixes TypeA, TypeB
entity type E2 mixes TypeA, TypeB

entity type C {
  typeE1OrE2: TypeA

entity type D {
  typeE1orE2: TypeB

Yes, you could do that but you wouldn’t have the Persistable apis at your disposal so this approach may not be very useful (how would you keep typeE1OrE2 up to date). What is your use case? can you use foreign key reference?