With tiamets splash damage? do other passive effects work with it?

aka frozen mallets slow? Lifesteal? Kennans passive shock thingy?

I went with I figured it probably didn't but thought it was at least worth an ask.

No. Only the actual damage dealt splashes.

aka only auto attack

Although not entirely sure, I think Ashe's Volley might proc tiamat splashes. (Btw it's spelled tiamat)

I've heard that if you get something like Madred's Razors or Bloodrazors, the extra damage from those items will splash as well.

I can confirm this. If your M Razor procs, it merely adds damage to your hit (rather than being a separate effect), which serves as the basis for your splash. Means you occasionally explode a creep wave.

Do they stack if you have more then one?

Tiamats do stack, and you can, in fact, cleave for more damage than you actually attack for.

It's why Tiamat Malphite is so fun.

Other on hit effects such as frozen mallet, bloodrazer, etc will not proc on the secondary targets, but any damage that adds to the actual damage (like sheen, madred's razor, madred's bloodrazor, not wit's end, malady) I have seen a twitch build of madred's razer + 4 tiamats based on the chance to deal 1000 bonus damage to everyone around on every hit. (but don't be that fail, please)

the problem with tiamat's is it's like 30% of the main damage to surrounding units (very small radius) but it is mostly useless against champions or even teamfights.

if you stack tiamat's, the damage adds (not unique) so you can with 4 tiamats do 120% to targets around your main target. BUT that only effectively means clearing minion waves fast

use it on champs that have a hard time farming. it really has no good uses other than that