Well, it depends.......
For the common "mixture charge" engine, where the fuel gas and air are mixed (in a mixer) before the turbocharger, then the fuel gas pressure into the mixer is zero, relative to atmosphere going into the mixer. The fuel gas is controlled in a gas train, comprising gas shut off valves and a zero pressure regulator. There will be a pressure drop across these devices and thus the pressure into the gas train will be above 20 milli bar, but could be over 100mbar.
A higher pressure is preferable, as the gas train will be smaller, as will the pipe diameter, and cost!
All pipework and any other devices (filters etc.) MUST be sized for the fuel gas flow at full engine power, this will prevent any pressure fluctuations during operation, which could lead to engine instability. As you say "ngas generators have sucked the main fuel line dry upon startup when it otherwise should have been properly sized". As someone once said to me "when all else fails, you have to engineer it!"
For natural gas fuelled engines, there may be need for a gas compressor to boost the pressure. In many cases, the utility pressure will be adequate.
For countries like the UK, the original gas supply was from coke works, and the supply pressure was low (20 mbar or so). When natural gas was introduced, the same distribution pressure was used. In some areas, higer utility gas pressures are available.
For biogas fuels, where are produced at low pressures from landfill sites or anaerobic digesters, a gas compressor is usually present.
There are other configurations of gas engine, such as those with direct fuel injection into the cylinders at several hundered bar, for these gas compression is almost always involved, as the utilities cannot supply at this pressure locally.
Each case has to be considered in its merits.
