If they HAD to feed a mix, probably something like Rabbit Royale. Though I think it's a bit too low in fibre to be 'ideal', but buns do seem to eat all of it, including the alfalfa pellets (they seem slightly softer than Russel Rabbit etc. ones)
But if they insist on a mix to give the rabbit variation, they could always buy a few different brands of pellets and introduce them gradually and feed a mix? Mine have both SS and Excel Lite as I find it hard to get hold on SS locally so want them used to Excel Lite incase I can't get any. This is also what they feed at my vet so if they go in and I haven't taken any SS, at least the Excel won't upset their tum.
The pellets are differently shaped and I imagine taste different, so would give the bun a bit of variation