I like Polyquaternium-10 but you need to keep in mind that those are like carbomers. Same INCI but the material might vary significantly! I used to have one that I used at 1% without any issues, and then I bought from another supplier and it behaves like xanthan gum (0.3% is a lot). Agree, there is additional benefit in using PQ 10 in products with hard to thicken surfactants. I discovered Quaternium 80 recently. It’s pretty strong and it’s a silicone!
I think it depends on the concentration of PQ-10. I’ve used 100% PQ-10, which thickens at 0.3%. There is also diluted version, which is easier to work with, but it is 10% concentration, so 3% of this version is needed to achieve the same effect as the 100% one.
No, there are different varieties. Some are more conditioning than others and some are more thickening than others. You would need to speak with the suppliers. Having said that, it’s a nice feature but I wouldn’t rely on thickening from PQ 10, because it’s derived from HEC which is a gum. So the thickening effect would be similar to what you get from gums, which I personally find not very pleasant. It’s a nice addition when you use hard to thicken surfactants, but I wouldn’t rely on it alone.