Is there any reason or advantage in changing gov'ts after turn 1?

Yes.
The advantages for each gov/strat remain until the first gov change. After that the advantages of a gov change needs to be weighed against the disadvantage of loosing 14% (or more if you're big) of everything you have that can't be put on the market.

other govts than theo and dict should be used since turn 1.

why not theo? It's good :p

Well you clearly forgotten about theo casher then.

depends if finances building as farmer or does cash start when low priv food sell price doesn't matter.