Does any one know if the Fantom 15 will drop right in, also, for the OS 15 engines do I need short shaft?

the Fantom WONT unless you cut the shaft to correct length with a dremel tool ( or something similar ). RC10GT will only take short shaft engines.

yup get a short shaft, you will be much happier you did.

Will the Fantom with a slide carb work?

the slide carb will require a 90deg change in throttle linkage end-direction. This may or may not be an easy addin.... Take a look at the throttle setups on folks MTs and TCs that run slides, they pull out, rotary pull forward to back...

just stick with rotary carb