Can a 5L plastic bottle be used as fermenter for the first days of fermentation? I'm looking then to transfer the juice to a 2L glass demijohn for the rest of fermentation.
If its food grade plastic then sure it shouldn't be a problem. I wouldn't recommend racking after a few days for no reason other than getting into a new container. You don't want to separate the cider (or beer or wine) from the fermenting yeast too soon. After a few days some of that yeast is likely to already be flocculating out, but its still active and helping with final stages of fermentation.
Large plastic containers (especially food grade containers) usually make excellent fermentation vessels if a bubble trap is fitted or any closure/lid/cap is not fastened tight but can allow CO2 out as fermentation proceeds. Failure to allow CO2 to escape will cause bottle inflation and possibly bursting.
Many home brews are made in 25 litre plastic buckets - a 5 litre version is just as good!