I want to make a thirst bar for normal sims that can be seen in the UI like the hunger bar.
Plant sims already have a fully functioning thirst system, and the package is very explicit, so I first try statistics tuning here. I deleted the blacklist for the bladder needs, and successfully created a plant sim with 7 needs.
However, while all the 7 needs function properly - they drop as expected, the UI for need bars only show 6. If I adjust the order of the needs, only the first 6 will show up.
How to extend this panel to include all of them?
Also, the situation becomes more strange as the panel automatically shrink if I blacklist more than 2 needs, but it doesn't extend if I add more.

The need-count is hardcoded into the flash UI, so you'd need to edit that instead, likely running you headlong into a conflict with the popular UIcheats mod. Sims can only display 6 "small bar" commodities, one "long-bar" commodity, and one "orb" commodity. Plantsims work around this by simply not having certain needs, thus keeping the count to 6 or fewer. Others work by similarly removing or merely reskinning needs (Mermaid hydration is just an Englishman with a dress).

If you wanted to be really clever, you could hack the code that determines which needs will be transmitted to just transmit the most-relevant needs, omitting motives that are not particularly relevant anymore, like "Fun" for a sim that has all the actual consequences of it blocked because they aren't even allowed to have Tense moodlets anymore, or just don't have that as a particularly pressing issue compared to more relevant needs. Like, say, if we had a better "Fire" system, a sim that was on fire would just have all of their needs replaced with the "Not Being On Fire" motive, since satisfying the "Not Being On Fire" need overrides all other concerns.