JR servos

Asa long as you watch the polarity of the plugs there is no problem. Match up black with brown. white with yellow. Red is in the middle so you can't do any damage plugging 'em wrong. Also what the direction of travel - it will be opposite. Pretty nifty to know if you need a servo to run a different direction in some applications. Like maybe a Y-harness for flaps or whatever

In two words none whatsoever. It is very common practice to mix and match servos from different manufactures. I should point out that the same is NOT true of crystals and there are many modelers who will not mix receivers and transmitters of different manufacturers (although I personally do and have never had a problem).

Like 2fast says, you can use them as long as the polarity is correct. Futaba connectors just have the "notch" on one side that makes sure they are installed the right direction.

Even without the notch, they are slightly bigger. If you only cut off the notch, they will be a very tight fit. Use some sand paper to make the fit better, and also bevel two corners to fit better.

I believe the initial post was using JR servos in a Fubar RX. In which case they fit without any mods at all. You just have to be careful of the polarity as has been addressed. With Fubar servos in JR RX all the above plug mods hold true.