I wanted to know what are the rules with PS4/PS3 Hard Drive RPM Speeds. I know there is 5400rpm and 7200rpm and Solid State Drives that are faster that what the standard console comes with.

Are certain HDD banned? Are certain HDD used only for speed running?

It seems a bit unfair if someone speedruns a game and gets 1st just because they can afford a faster HDD..., i don't know.

Each game generally has its own individual ruleset, as it would be impractical to impose universal rules for all games on a system with parts that can be modified such as this when it may affect different games in different ways. Consult your relevant games' leaderboards or the mods in charge of them.

"It seems a bit unfair if someone speedruns a game and gets 1st just because they can afford a faster HDD..., i don't know."

Many games are "pay-to-win" like this. Many games.