I'm replaying DCL #7 right now and found that (on this map at least) Trad-Piety mix works quite well.
I went Trad Opener -> Wonders (for ToA) -> Piety Opener (hard built Steles so Legalism not so important) -> Organised Religion -> Monarchy -> Reformation -> Finish Tradition.
Gave me a strong religion, there were plenty of river tiles for growth after Civil Service, early religion meant happiness buildings (could have had both mosques and Pagodas but wanted to try Religious Art for once). World Church gave a decent amount of culture before Trad finish.
I still feel reformation is too far down piety (4 policies total) to work well with tradition.
The main strength of tradition is to free up some early hammer with free monuments and very fast aqueducts.
I feel Piety would mix better with Liberty which can be stoped after taking 3. It also work better with Piety's bonuses if you set up a wider game than 4 city tradition. The only other problem is that then taking reformation for your 7th policy runs the risk of getting a poor one...
Well on this map the problem is that you don't have room to expand so lib/piety wouldn't be that interesting on this particular one.
Yes 4 cities Hardly justifying lib/piety.