Look at the code in mycroft-core/mycroft/tts
That is where different TTS that Mycroft current supports. There isnt Cepstral/swift support yet, so you or someone have to make that first. You can look into the others TTS to se who they are made and get inspiration.
As I see the Cepstral/swift have a API that can be used if one gets a API key so adding support for that in Mycroft seems posible.