While this mod lets you resize sparks, faction troops and regular troops as intended. This mod cannot recognize other types of troops added by other mods and i couldnt find a way to get around this via some simple ini edits so i guess to fix this the intended way is through SDK kit and that is way beyond my scope. I did post a comment about it but Musashi hasnt responded...and it has been some weeks already.

So i guess we are on our own and i did found the solution that does involve ini edits and its so simple.

1) This mod uses, in the documents a folder called XCOMBODYSLIDER.ini, it contains all data for resized troops you already have.

We are gona hijack this feature, because it uses name recognition to rescale models
(you might get where this is going)

2) We open it and there should be a list of all troops
(example)
CharpoolSoldierSizes=(SoldierName="Asari Supersoldier",Size=1.300000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Mike Adams",Size=1.200000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="SPARK-408",Size=1.200000,HeadSize=0.000000)

Now, since this mod works on name recogniton to apply scale, copy and paste one of these lines and simply rename them to a name of your alien soldier and repeat till you are satisfied with options.

3) Rename the name in the quotes so the "Mike Adams" to "Serpent Topsnek" (Or whatever you want) and adjust the value in the "size" to value you want.

4) In game (in the armoury), rename your intended target to the exact name in the ini and he will get rescaled when you pull back to armory.
(The first name and last name must be the same as in the ini, nickname is irrelevant)

5) Make an array of names in the ini to cover different sizes and species, now you can be creative and actually give them names but for sake of simplicity i named vipers i want to resize as "Serpent Omegaone", each number means increase in size so i can get king viper sized anacondas. Or smaller
(If you are lazy, you can copy and paste these, dont worry it will be formated correctly in the ini)

CharpoolSoldierSizes=(SoldierName="Serpent Omegaone",Size=1.150000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Omegatwo",Size=1.250000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Omegathree",Size=1.350000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Omegafour",Size=1.450000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Omegafive",Size=1.550000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Littleone",Size=0.850000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Littletwo",Size=0.70000,HeadSize=0.000000)
CharpoolSoldierSizes=(SoldierName="Serpent Littlethree",Size=0.600000,HeadSize=0.000000)



Result :
(See, Serpent Omegatwo is larger, Serpent Littlethree is really small)

https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2498196628




TLDR
Mod assigns sizes to names, you can hijack this and copy and paste entries and replace the names with the names of your desired target, then you just need to name your alien/geth exactly as in the ini and you will fool the game to rescale its model.

Create a wide array of sizes in the file if you want, make them actual names if you can actually remember them.