For long and short at the same time.
The way I currently understand it is that it's against SEC rulings
to have interests in opposite directions. (i.e. crossing interests.)

Slippage should probably raise an exception if there are orders
in opposite directions.

For the question about partial fills, etc.
The intent of FixedSlippage is to provide a slippage model that
just fills orders without any considerations.
With the hope of making other parts of the code base that included
slippage in the chain more testable/paper-and-pencil-able when testing
because the slippage would fire immediately with Fixed.