Is it permissible to pay a fixed monthly amount to the electricity company for estimated consumption (the "practice" system), rather than actual consumption, until meters are installed?

If both parties (the consumer and the electricity company) agree to estimate the consumption until the actual meter is installed, there is no harm in that. The gharar (uncertainty) here is overlooked because it is minor or because it is difficult to avoid. Not all gharar is a cause for prohibition. Minor gharar or gharar that cannot be avoided does not invalidate the contract.