When I connect an outtake directly to one of Krastorio 2's Huge Storage Tanks (3 pipe connectors at each side) and try to connect 6 regular pipes to the outtake, i get an error message that this part can only get connected to 1 duct part and not 6, and the outtake gets removed.

https://imgur.com/a/W4rtFZM

Correct, this is not legal:

5 pipe + 1 storage tank that count as a non-duct-part, what u try to connect is the 7 element.

U should connect duct parts only to other duct parts, or u should use the two segments in and out to do that.. but they are not fluid outlets :)
The fact that work with 5 pipes is a glitch that I can't solve,

Oh alright , never mind then, I was actually wondering why that worked in the first place, so that answers at least that question.

On another note, I tried getting the maximum stated 72000/s pumping capacity out of curiosity with different setups but I never managed. How would you achieve that pumping capacity?

Eheh pratically only in creative, u should have an huge amount of fluid on sender and a big empty receiver