Not sure if it's possible glueing them to a 1/8 Truggy rim, but I also searched for non-glued 1/5 touring car tires, couldn't find any, only 1/5 rain tires.
Ungluing them is probably not going to be easy, I won't do that anymore, did that years ago with 1/8 tires, tried both the acetone method and oven method, it's a mess and smells like hell.
If you put in softer foams, you'll probably also have to turn down those additional plastic parts of the rims like I did with the larger rims, otherwise bumps will probably "go through" the foams and hit the rim.
Here's what I mean:
On my 5.4Kg Truggy they didn't make much grip and didn't reach operating temperature, I guess the car was too light. Also the foams were a bit too hard like you already noticed. With the 8Kg Twin Truggy they are okay when they reach about 40-50 celsius. From what I've read in largescale forums, they need a 10Kg car and temps of 60-70 Celsius to get real sticky like glue.
Maybe try the new GRP 1/5 tires, they are supposed to be about 10% lighter than the PMTs and GRP claims their rubber compound doesn't rely on temperature. Not sure if that is true though.