I would tie it to your choice on dispelling listed in Arcane Co 6.3.8. If Arcane/hybrid are more powerful as in option #1, use the lowest racial RR mod, if option #2, average them, and if Arcane/hybrid are fragile(option #3) add them as you did in the OP.
For Arcane I am on your side- for Hybrids though is not such a rule for race in RMSS/FRPG. And if Arcane is an own realm, (option 1) that does not automatically mean, that hybrid is also more powerful.
In addition the wording of 6.3.8 is as follows"
you could also argue, that arcane has no racial modifiers because its a new realm where nobody has inate RR bonus same as no dispelling from other 6.3.8 • DISPELLING ARCANE MAGIC
There are three different ways to view the cancelling and/or dispelling of Arcane magic (note that the same
arguments apply to cancelling and/or dispelling hybrid spells). The GM should choose the option below that best fits his view of his world.
Option 1: Arcane magic is inherently more powerful and different than any of the magics presented in Spell Law.
Thus, it can only be dispelled by an Arcane dispelling spell (found in the Arcane lists like Realm Law). If no option is specified by the GM, this option should be presumed to be in effect." So nothing about race modifiers is worded here...realms are functioning vs. arcane. You could use the least bonus (or penalty), an average of all or add all together, because in 6.3.5: "
RESISTANCE ROLLS
When resisting against Arcane spells, the RR is modified by the sum of the target’s Empathy, Intution, and Presence stat bonuses." Thats not the average. But RR for races are already tripled for one realm so I would advice to divide all RR racial boni trought 3 and then add them or use the average (mathematically the same).