If you mean by rivalry that we should go about our day killing each other simply out of our own blind selfish pride over our differences, we may have to go about attacking everyone for just the same reason. I doubt very much you'll see that. That stubborn pride that you mention only means we prefer to keep to ourselves or our kin for the most part. That, by all means doesn't limit or restrict us from being an active part of the community'
If you wish to see the Dwarven rivalry at it's best, watch the marketplace and auctions closely. I often find myself being out-bid or undersold by the very Dwarves whose company I enjoy. Then, again I do the same. Now, haggling with these guys trying to get the best part of sale has been one of the funnest aspects of this game. This is not limited to Hammerwielder vs Stonebreaker however. Just a friendly rivalry between each other and is seldom anymore offensive then an arm wrestling match. You beat me once doesn't mean you'll beat me twice.
The two races are different, but as you said neither are from the areas presently in game. So one would tend to mingle with others of similar traits or skills. Being both of the Dwavern race in general and having a common interest in the ore, metal, and crafting trades. I think in reality you would find them working together out of common goals, and interests with each other more then other races. The rivalry you suggest is of some separated, unschooled, secluded sect that had been brought up in generations of hatred for some long ago forgotten offense.
The term "Brodr," Broder," or "brother," are all shorts, slang for "brotherhood." Under the meaning of those who share a common bond. This does not imply by any means that we are family.
I don't use the term "Brodr," but I do use "Brother" as a greeting or a title of respect to other dwarves regardless of their race. I for one see no offense or difference to that vs "Mi-lady, Lady, Miss, Sir, Mister, Master, Lord," or any other title used to greet or address someone with a form of respect. Only that I limit it's use to the Dwarven races, my guild, and two other characters who arrived in Hydlaa near the same time as I and we grew together through our infancy. [who are not dwarves by the way]
You did make a point that once the graphics are added to Hammerweilder's characters we would see a change in how many characters are played. Of this you may be right. But, keep in mind that many of us have already bonded our friendship of each other. So it may not be as big of a deal as you propose. There will be some offspring guilds that will form up exclusively for either Stonebreaker's or Hammerwielder's. Making room for the type of rivalry that you have mentioned. But, through it all I think you will find that the Dwarven guilds that exists will weather that storm well.
That, all being said if you wish to play out the rivalry that you wish to see, that is entirely up to you. As an individual dwarf that is at your discretion. I would advise as an elder Stonebreaker. "Dat, yah be aware o' where yah be scratchin' yah line. B'cause, yah may just find yahself bein' held ta it's confine's."