Ah ha! I knew posting this thread
would come in handy in the future.
The chart I linked to in that thread
shows which Connections can mate with which.
If you need help interpreting that chart, I'll make it simple for you:
Akais, Uratamas and Entamas can only mate with their own types.
Akai can connect with any type, but only mate with other Akais.
Uratamas can connect with Japanese Tamagotchis, but only mate with other Uratamas.
Hope that helps!