The FSB Pro Expert Advisors can execute its orders FIFO or in FILO order. See around line 90 of the EA:
// FIFO (First In First Out) forces the expert to close positions starting from
// the oldest one. This rule complies with the new NFA regulations.
// If you want to close the positions from the newest one (FILO), change the variable to "false".
// This doesn't change the normal work of Forex Strategy Builder.
bool FIFO_order = true;
However, these rules are only valid for the orders of a single expert. It is useful when the EA adds or reduces a position.
On the other hand, there is no mechanism to consider orders execution between different experts. It is simply not practical because we cannot suspend th entry or the exit of one strategy because another strategy has some position.
Because you named the topic " How FSB - EA handle orders in a FIFO environment...", I want to add that this is how MT handles the orders. We cannot do anything to change the regulations or the brokers' settings.
In order to have reliable trading, you need to trade a single expert per symbol in FIFO environment.