My first question is - why does having RAF matter? If you have full heirlooms, then using heirlooms is better than RAF. Full heirlooms yields 55% bonus exp where as RAF gives only 50% bonus exp - but there's a catch! If you have RAF active you get 50% bonus exp even if your heirlooms give 55%, this is the way it currently is as of 8.0 - not sure if 8.1 changes this. I tested this after I noticed 1 of my 5 team was leveling faster and noticed the one that is not linked in RAF is getting more exp than the ones that have RAF.

Otherwise, is this starting at minimum level (1, 20, 55, 98) or will these be boosted (110)? If you are leveling from minimum level then DK + DH would go together, as 55-98 could be done with 2x DK which would minimize the number of required total characters.