Booking ahead for large vehicles

If your city allows you to schedule trips, then yes, you can schedule XL vehicles in advance.  Keep in mind several things.  Getting multiple vehicles in advance is NOT possible, only one at a time.  Also keep in mind that many XL's can seat 6 but it's a squeeze, so if comfort matters, you may want to plan for 5 passengers (plus the driver) to a vehicle.

More importantly, you can only request ONE vehicle at a time, so if you need multiple vehicles it will be a problem.

Lastly, keep in mind that using two regular Uber/Lyfts only costs slightly more than one XL, so there's not a huge savings by going to XL.  It's a matter of convenience.