4 A 380 stands at T3 are already there. BA has exclusive use of T5 and will continue to operate some flights through T3 as T5 is too small for them from the outset. No other operators will be able to move A 380s into T5.
Nobody should worry about the A 380 being too big for BA. They could profitably use half a dozen today. If anything , as above, the current model is too small and it doesn't come into its own until stretched.
As far as Terminal handling is concerned , it is no worse that the simultaneous arrival of a 747 and 737 and that's happening all over the world all the time, so no worries there either.