What about civs that only primarily develop one city? Such as Venice (or my friends who like to play one city challenge).
Gerrymandering
I don't see an issue. The Prestige modifier is the same (up to 100% for every opposed and favoured reform you implement), it'll just all concentrate according to the interests of one party. So basically Cuba.
For Despotic Monarchy/Republic, I believe I'll simplify. Let me outline the influence of the legislature on your reform costs as it stands:
For Republics, each Senator will increase and decrease the reform costs
of a particular category by 100%, depending upon the faction that senator belongs to (e.g. Civic Faction senators will increase Govt Reforms by 100%, whilst decreasing Industrialist Reforms by 100%).
--Oligarchic Republics have 1 Senator per pop in the Capital
--Democratic Republics have 2 Senators per city
For Monarchies, each Political Party will increase and decrease reform costs
of all categories by the proportion of parliament they have, for every opposed or favoured reform you've implemented (e.g. if the Clergy is 100% dominant, each Religious Left reform will decrease costs by 100%, whilst each Government Right reform will increase costs by 100%).
--Parliamentary Monarchies may deal with Conservatives, Liberals, Libertarians, Fascists, etc.
--Absolute Monarchies may deal with Nobility and Clergy
The idea for Despotic Republic was 1 Senator per category, effectively negating any affect on reform costs. However, that's probably unjustly superior to the other forms.
Hence, I'm thinking I'll simplify Despotic forms of government to a base modifier per city/pop (possibly both). So early game Despotic governments are great, though with no opportunity for cost deflation, and as the game goes on you'll be incentivised to adopt a more sophisticated form of government.