I work for a P@H store myself, the only reason i can think of that he wouldn't recommend bonding with Stella is maybe they have tried mixing her before and she is aggressive towards others or perhaps she has suffered an illness in the past such as snuffles or syphillus that is likely to re-occur and therefore you cannot risk mixing her due to possibly passing it on? Ask a different member of staff if they have more information on her (ask the staff rather than managers as it is the staff who spend time with the animals and care for them, they will have a better idea on their personailty etc).
I don't see how trying to bond Stephen with a new bunny will stress him out, rabbits are generall sociable creatures. I recently lost a rabbit (Alfie) who had bonded with another (Ava)... Ava became depressed and ill after Alfie passed and i was worried she wouldn't survive, she had just been neutered too. 2 days ago i took a rabbit home that had been in our adoption scheme at work and Ava is back to her normal self, happy, eating and playing with her new boyfriend (Big Ears). In my experience, bonding Stephen with another rabbit is a great idea!