Hi Jennifer: Welcome to Permies!
There is nothing you can burn that is to hot for an RMH. Back east, folks who have access will put anthracite coal in to burn.
Black locust, eucalyptus, orange osage, or any hard or soft wood a
Rocket mass heater will happily burn them as long and hot as you can stand it.
As far as design modifications; Any proven rmh build style will work fine. If you have the finances then using the newest product innovations, like ceramic fiber blanket for a 5 minute riser and ceramic boards for your core will give you more heat sooner than a old school heavy firebrick core. But A heavy firebrick core will work just fine for many many years.
Do you have a copy of the RMH builders guide ? Highly recommended! Easily available on Amazon.