As a Paris resident, I think I have enough authority to answer these questions.
First, some basic info. Bois de Boulogne is a vaste woods region in the west of Paris. The shemales can be found along the Allée de la Reine Marguerite, from the Rond Point Boulogne-Billancourt till Allée de Longchamps. The highest concentration of shemales is around Racing Club de Paris.
The shemales in Bois de Boulogne are mostly south-american (66% to 75%), with a minority of maghreb shemales from Morocco, Algeria and Tunesia (33% to 25%). There are some active around the noon break (12-2), then the action starts after 5pm.
After sunset the Bois de Boulogne can be dangerous. Not all of the present persons are sex workers, some of them are aggressive thieves just crossdressing, who never had the slightest idea to engage in prostitution. Their only reason of their presence is to rob you.
For the sex, it is a fuck in the woods or in the van of the shemale.
There is no public transportation nearby. The closest you can get is the metro station Porte Dauphine or Porte d'Auteuil, but you are still at least 1 or 2 kms away from the action. Walking in Bois de Boulogne after sunset can be dangerous. Taxis will refuse to go to Bois de Boulogne after sunset.
My answers are in the text highlighted in red.
One remark. The french government prepares a law criminalizing the clients of prostitutes, after the Swedish model. The law proposal should be discussed next week in the french parliament. If it passes, the action in Bois de Boulogne will be dead, but otherwise the law will be inapplicable. I also expect that large parts of it, if not the whole law, would be cancelled by the french constitutional court.