By balanced I mean that when this slime splits, it pays a price (depending on what variant rule you use).
I was trying to bake up a slime encounter suitable for low level PC's. I saw the String Slime and liked it, but the fact that it's splitting magnifies it's action economy with no penalty (other than risk from area attacks) bothered me, as did the fact that slashing always splits it and other damage types do not; this could make certain players feel singled out, or fail to teach the "split" mechanic if it is never triggered.
This is my (perhaps overly complex) response. I'm planning to use the 3rd level Slime Mold as an encounter in the beginner box temple. I've run it against the Iconics and they generally come through fine, with maybe one character getting seriously injured but not downed, though the casters do typically use some spell slots. I suspect normal players will fair worse, but probably not to much so - the need to use a reaction to split curbs the craziness, as does the fact the new slimes are either lower level, or are stunned. I plan to use the "resize" rule variant myself.
As you might see, I only have stats for slimes up to level 5. Levels 7 and 9 will continue the trend of being larger, slower moving, and harder hitting, likely also gaining reach and the ability to split into un-equal sized slimes and / or more than two slimes. I might add some other oddities suitable for a mega-sized "collective life form" that hint at emergent (and very alien) "intelligence".
Rules are linked as imaged from my Foundry screen. Sorry about the awful rule formatting, if there's a better way to post Foundry sheets here, let me know!
https://imgur.com/a/O8f1AOC
So I used this for my Trouble Under Otari / Beginer box group. Single level 3 slime mold vs party of 4 (armor inventor w/ gada, guardian w/ sword & board, water kinet, and commander with pike).
The critter worked really well I think, though seemed a bit high threat for an 80xp encounter. One party member went into the temple and drank from the bowl (acing her fortitude roll, though I was gonna give her the blessing anyhow since she's a hydrologist azarketi), at which point I had the slime ooze out of the statues mouth to start combat. (I also had the "out" in the back of my mind that the temple would use the water bowl to heal them and the slime push them out the doors and seal the temple, if it was a tpk.)
After splitting in reaction to being attacked, one slime downed one party member (the kineticist who was at the alter) and the other nearly got two more (lucky crit hits), but the party quickly knocked one split under 10HP. The more injured slime re-merged and the fight went a couple more rounds, but I didn't split again to maintain medium size (though I did use the reaction once without splitting).
Needing to use reactions to split but then having the reaction negate that physical damage entirely regardless of type seemed to work well, but the merge action was awkward because both times I used it it amounted to one slime moving to "Flank" and then merging back into one that didn't move. I think I may add a note that the non-acting slime is the one that moves (which would have been possible, even better tactics, but I forgot to do so).
I think I ended up using the split reaction like 3 times and the merge twice (once just for fun offence, once to absorb a slime that was under 10 HP so couldn't use it's split reaction any more). The merge ability really made sense when used by a slime that had taken a beating and would no longer be able to use splitting to ignore damage.
I think the default string slime it was based on would be MORE deadly, so probably also shouldn't be an 80 xp fight vs 4 level 1's. The changes in tokens and nuttiness with initiative ordering made it a pain in the ass to run though. I'm working on cleaning up the text a bit, would love to run it again or somehow get other action reports. One thing I did already was to drop all the attacks to match the creature build "moderate" level - the string slime is one higher, but landing multiple crits from that thing on level 1 party is HARSH.