The vehicles that Bee Network routes are decided by TfGM. They include it in the contract for them, I did get sent some photos of Tranche 1 routes when speculation as to what would do what route was rife.
It's why you almost never see Bee Network routes with random fleets, and those that you do see that on are usually a temporary basis of it - for example, I believe the 41 will be doubles when everything arrives. That's not for sure, but I'm almost certain I heard that somewhere.
The 52/53 are booked as Evoras and E300s. It seems TfGM are classing the two types as so similar that they can be interworked on contracts. It makes sense I guess, they're similar size and types of bus overall.