Am I required to open a separate bank account for surplus funds?

Only if you are spending your surplus funds on non-reimbursed public-office related expenses. A surplus funds bank account is a perpetual account each time you run for office and are left with surplus funds, they may be deposited into the surplus funds bank account regardless of which office you sought. (e.g., Surplus funds from a city council campaign can be deposited into an account that contains surplus funds from a state legislative campaign.) Only surplus funds can be deposited into the surplus funds bank account!