Hypnosis on Gengar over Substitute would further contribute to setting up your Sweepers.
Skarmory and Zapdos are both a pain in the arse, because together it walls your team pretty darn well.
Heracross, Ice Punch Gengar, CB Normals / Curselax present problems, too. Jirachi and Celebi come close as well.
You'll have to break your RBY-only restriction, because Steel Pokemon seems to help a whole bunch for your team's defenses. Registeel with Thunder, Thunder Wave, Counter, Earthquake / Explosion perhaps. Thunder spoils Skarmory's attempt to stack Spikes; T-Wave helps both Snorlax and Slowbro in the offensive, as well as crippling threatening Sweepers; Counter EQs / Curselax's moves; and Explosion / Earthquake to break Ttar, Jirachi, and Raikou's sub. Possibly over Dragonite, since Dragonite can't sweep in a team that lacks fundamental coverage to popular Pokemon anyways, and doesn't gain much from Registeel's support.
Slowbro needs Ice beam if it wants to be a functional water pokemon, and Ice provides a reliable offense against Salamence, Celebi and Zapdos. Although, Fire Blast seems very appealing for dealing with Steels and such, Surf is necessary to provide better coverage.