Here is a link to that engines owners manual which answers most of your questions.
http://www.toprc.co.nz/GMS%202000%20ENGINES%20OWNER'S%20MANUAL.htm
A quote from the manual:
BREAK-IN
Your GMS engine has a closely fit piston which requires only 15 to 30 minutes running before it will idle reliably. No special fuel or propeller is necessary, but it is recommended you use the fuel and propeller you intend to use in your plane. Take care that the engine is not run overlean (leaned in to the point that it loses power).
The recomended prop range for this engine is 8x6 to 9x6 and me being who I am would even try a 10x5 based on those numbers just to try to get a little more pull with less top speed on a trainer. If it loads your engine to much (not getting full RPM out of it or slow spin up) you could try a 9x5 or a 10x4. This manual also says something about not using nylon props due to excessive vibration... I think that is a load of horse stuff but it's your engine. As far as fuel is concerned I like 15% nitro and 18% Synthetic oil with a splash of Sig castor oil for good measure (roughly raising the oil content to 20%)... That is me though.
Another quote:
GLOW PLUGS
This series of engines all use the long thread glow plug. We recommend medium heat range plus.
As far as glow plug I like OS #8's.
Another quote from the manual:
FUEL TO USE
Model airplane fuels are a mixture of methanol, oil, and various additives to improve ignition, add power, and cut down carbon deposits. It is important to use fuel with a castor oil and/or synthetic lubricants, otherwise, your engine life will be drastically reduced. Nitro is another fuel ingredient that is often used. Its function is to "advance the spark". To determine if you need any, or more nitro, start your engine, open the throttle wide open and set your high speed needle for maximum power, then back it out slightly. Do all this with your booster battery connected. Now remove your booster leads - if your motor slows down, more nitro is indicated. Conversely, if your motor speeds up, there is probably too much nitro. We recommend high quality commercial fuel with 10~15% of nitro.
There description of what indicates a need for more or less nitro can also be and more likely be an indication that your tuning is off or your glow plug is going bad.