I currently have 98 composite 2 walls that I converted to lvl 3. I will assume that the maximum allowed by kix before I upgraded to lvl 3 was 98 walls.
Is it possible now to remove other lvl 11 walls to make room for more composite walls?
Because composite walls count as several "regular" walls, yes, there is a hard limit on how many you can build. And yes, you would have to scrap regular walls in order to build more composite ones up to the max allowed.
Walls and composite walls share the same pool count. At most you can have 600 "points" worth of wall pieces. Each normal wall counts as 1 piece, and each composite wall counts as 4 pieces. You can mix and match as you see fit and you can always scrap and rebuild them if you want one of more or the other.
Upgrading your OP unlocks them. And then you build them the same way that you build a normal wall, provided you have the capacity to build one. If you don't have all 600 walls built you will need to scrap some first. 4 wall = 1 composite wall
Remember you can scrap walls row by row, if you have normal walls to get rid of (normal walls are a total waste of time tbh)
If you don't have all 600 walls built you will need to scrap some first. 4 wall = 1 composite wall