You can download the FS9/FS2004 Nukem effect from SurClaro in the FS2004 Effects. It works in both FSX and FS9 perfectly.
You cannot download Microsoft Flight Simulator X.
You cannot download Microsoft Flight Simulator 2004, you have to buy it.
Yes, you can download add-ons for Microsoft Flight Simulator 2004.
The download for the Antonov 225 for Microsoft Flight Simulator X is is in the link below this question.
There is no need to buy Microsoft Flight Simulator X: Deluxe Edition as Microsoft Flight Simulator X: Acceleration contains everything from Microsoft Flight Simulator X: Deluxe Edition and more.
I have found a download for the Embraer 135 for Microsoft Flight Simulator X. I have put the link to the download below this question. To download you must register with the website but registration is free.
Download fspassengers
No, you cannot use the add-ons for Microsoft Flight Simulator 98 in Microsoft Flight Simulator X.
No, Microsoft Flight Simulator 2004 add-on aircraft cannot be transferred to Microsoft Flight Simulator X.
Flight Simulator X is by Microsoft, the full name is Microsoft Flight Simulator X.
Microsoft Flight Simulator X: 18 Microsoft Flight Simulator X: Deluxe Edition: 22 Microsoft Flight Simulator X: Acceleration (add-on): 25 Microsoft Flight Simulator X: Gold Edition: 25
Microsoft Flight Simulator was created in 1982.