RE: JR Tx compatible with a Futaba Rx?
As long as the cables match.. + voltage supply and - voltage supply to the appropriate power leads, and the colored signal wire goes to the appropriate signal wire match up on the extension, or in that slot on the receiver, it will work. Note that some servo leads have the + lead in the center while others have it on the outside. Just note that so you don't get them reversed, and it should be just fine. You can reverse them if you wish so that the +'s and the -'s match. (I use those symbols because some may be colored differently, for instance, brown or black will indicate a - polarity, where yellow or blue may indicate signal wire).
I believe Minnflyer or RC Ken has posted a picture denoting the leads and the colors for various types of servo's somewhere here on RCU.