G'day Mate,
I have a Saito FG21, which is the equivalent to a 125 Glow, but slightly less power, I run a APC17x6, on a Spacewalker, that plane loves the 17x6, I have tried a 16x6, and a 16x8, BUT on a Decathlon, which is designed for aerobatics, I would try more pitch, like a 16x8, or 16x10, this plane needs more speed.
The plane, & the way you fly it will determine the best prop, NOT RPM figures. By the sound of it, your 125 is not run in yet, & will develop more power as it runs in.
Fly it & forget theory, don't over think it.
Cheers