I know FPS Creator inside out , I know most of the history behind FPS Creator Reloaded/Game Guru (yes it's the same engine) and I have spent 25+ hours into trying GG out - so I'll try to answer your questions as best and objective as I can.
Quote: "
They say now you can make huge levels with GameGuru. As you are well aware, the limitations to levels with FPS classic are 40 squares by 40 squares (up five and down five), with a virtual memory cap of 180 GB. My question is how does GameGuru differ from the fore mentioned limitations that FPS Creator classic has?"
That's something where GG excels compared to FPSC. You can indeed create outdoor levels that are large enough for any kind of games, afaik it's 512 by 512 squares.
Quote: "My next question is, can I populate indoor and outdoor levels with more entities then I can with FPS classic creator?"
Definitely. Performance is always an issue (with every game and editor) but at least you can easily put 50+ characters on the map without crashing the engine. You won't be able to spawn too much at the same time without affecting performance as well as character animations but it's still a big improvement over FPSC.
Quote: "My next question is can the skybox or its equivalent be changed from one level to the next?"
Yes you can. Compared to the abundance of skyboxes that where created for FPSC, GG is far behind but the engine makes up for it with lightmapping tools that let you create any ambience with ease (and a little practice).
Quote: "All the available options has the same descriptions. Does anyone have any information what the differences are from the various options and why there is no GameGuru - 3 Pack."
There is no difference in the content, you are getting additional steam keys to give to your friends (or fellow developers). If you want to play multi-player maps with your friends they'll need GG as well, otherwise I see no need for additional copies.
Quote: "My next question is can GameGuru be installed on a machine that has FPS Creator classic already installed on it without either application corrupting the other?"
I never had any problems after installing GG on the same PC as FPSC. Just don't try to run both at the same time.
So is Game Guru worth buying?
Personally, as an FPSC addict I knew I had to try it out sooner or later so I bought it on the Steam release day. But I was overall very disappointed for various reasons and went back to FPSC. However I already converted a lot of GG models and textures for FPSC, so I don't regret having spent 14.99.
I hope my answer helped a bit.
In case you find my grammar and spelling weird ---> native German speaker ^^