The 26cc RCGF and RCG engines work good with a 18x8 propeller. Using a smaller prop puts the engine at risk of exceeding its maximum RPMs rating. Using regular pump gasoline with some ethanol in it and that odorous butane like stuff in it too, I got these numbers.
I was running a RCG 26cc gas engine that is shown in a thread over in the Engine tachometer readings category.
I nevewr tried running naptha based fuel (ColemanCampFuel), so I don't know if you need octane booster or not. I do know others have used the fuel in othergas engines before without mentioning usingoctane booster.
I have used a infrared digital thermometer I got fromHarbor Freight Tools OK on many engines.

Propeller RPM numbers so far gathered:
Master Airscrew 16x8 composite prop 8,480
Xoar 17x8 wood prop 7,780
Master Airscrew 18x6 Classic composite prop 8,130

I did some further tests and got these results using some 18x8 props
Master Airscrew Classic 18x8 prop 7,095
APC 18x8 prop 7,095 (WARNING dangerousto do as the prop hub is weakened by the drilling for the prop screws).
Vess 18x8 Sport prop 7,225
Xoar 18x8 prop 7,180