Thats what I was scared of. Made a few more tests to be sure, counted the number of monsters in a specific area of Blood Moor.
Testing hidden as small font because most people do not want to read it anyway.
Counting 1 ||| nummon = 1, mon1 = zombie ||| Number of monsters counted: 23, 26, 20-22 (2 were on the edge of my chosen area) |||average = 23,3333
Counting 2 ||| nummon = 5, mon = zombie, and the 4 different Goblins with SparsePopulate = 5, 1, 5, 5 (Sparsepopulate should have been 1, but since goblins didn't matter this didn't matter as well) ||| "Rarity" value was supposed to be the same for both, zombies and goblins. Instead, i made an error and the Rarity of Zombies was 1 while the Rarity of Goblins was 2. ||| Number of monsters counted: 1, 5, 2, 5, 2, 0, 0 (+1 Goblin), 3, 1, 0, 5 (finally again!) (+2 Goblins), 2 (+2 Goblins) ||| average = 2,1666 ||| The expectation was 1/9 of 23,333 = 2.5926 (1/9 = Number of Zombie Entries * Zombie Rarity / Number of all Entries * Corresponding Rarities)
Counting 3 ||| nummon = 5, same as above, but with Goblin Rarity = 1 instead of 2 (therefore now same as zombie Rarity) ||| Number of monsters counted: 3 (+1 Goblin), 6, 2, 6 (+2 Goblins), 5-6, 5 (+1 Goblin) ||| average = 4,5833 ||| The expectation was 1/5 of 23,333 = 4,6666
So it worked as expected, adjusting MonDen shouldn't be too hard to keep it close to the original. I hope MonDen has no quirks.
Conlusion: If you have 5 monsters in one area, and add 4 Goblins to it, the MonDen needs to be theoretically increased by a factor of 9/5 7/5 (if the rarity of the monster is 2 and the rarity of the goblin is 1), if the density is supposed to stay the same. NumMon needs to be increased from 3 or whatever the value was, to 9. If the original value was 3, it results in no longer only 3 different kind of monsters be chosen for the level. Which might be an unwanted effect, but I don't think there is a workaround for that. Some of it was tested and some of it was my conclusion but without explicit testing.
Edit: I've got a new question. Is it possible to make the goblin attackable during his escape (animation)? I have not worked with monster animations before, and after searching through .cof (Coffee CofEditor), skills.txt, and other .txt files I have not found the thing that makes him untargetable. I'm kinda not expecting this to work because the sequence has two parts (escape/ invisibility), but since I don't even know how to make him attackable during the whole sequence, maybe there is something else I don't know about.
(On a side note: skills.txt in "Do it yourself - All goblins separated" seems broken; and the get hit/ waiting animation of Gelantinous & Gem Hoarder were not transparent here, but it was easy fixible with the Coffee CofEditor. I don't know if Gem Hoarder is supposed to be transparent or not)