Stacking Healthstones were indeed something we did back then. Different Warlocks running the same raid would deliberately put different points into the talent so they could stack the stones. If you ran alone, it was usually smart to dock 1/2 points, as most other Warlocks went 0/2 or 2/2, so this meant more stacking mismatches. If you're leveling with a friend that wants to run that same class, mismatch the ranks so you can stack stones. You can argue this is an exploit, but since they still share the same item CD, no one complained about it and Blizzard didn't care for years until eventually fixing it after Classic.
I do wonder if the CD groups are something Nostalrius is doing correctly. Potions+Healthstones should be in 2 different CD groups, usable together. Mage Mana Gems should be in the second group. Many special items, such as Magic Dust, Slumber Sand, and any grenades, should share CDs. Trinkets are usually unique to themselves, but some also shared CDs. However, Felwood plants should be in the Healthstone CD group, not the potion group. Farming these is how you could double potion without Healthstones:
http://www.wowwiki.com/Whipper_Root_TuberThere were also these, if you wanted mana:
http://www.wowwiki.com/Night_Dragon%27s_BreathAnd these, if you REALLY wanted mana:
http://www.wowwiki.com/Demonic_RuneThese all shared CDs with Healthstones and each other, but not regular potions. They were useful if you weren't playing a Warlock. Basically, items branded as "potion" were 1 group, and things branded as something else were a separate group.