Rubies, much like emeralds and sapphires are found near the diamond spawning rates, so if you don't find that many, try digging between 5-12 y. ALSO sapphires are ONLY found in ocean biomes, and emeralds are ONLY in the extreme hills and EH edge biomes. I believe rubies should spawn in the other biomes, like plains and biomes, trust me, i have over 300 rubies in my ssp game, so it's not really an issue, i also believe that they are Ore dict friendly, meaning that the RP2 versions should be compatible.