I have spectrum rx's, both powersafe and non power safe and use a combination of digital and analogue servo's in all of my jets. Some jets even use plain old 3 pole cored servo's on things like valves. I have pylon racers and sport planes with spektrum receivers that use 30 year old analogue servo's. Some pylon planes have a combination of digital, coreless, and 3 pole servo's with high frequency vibration screaming engines on them.
I have never seen or heard any warnings about mixing servo's in instruction manuals, online or otherwise. Spektrum or JR 2.4 receivers. The only thing I ever had a problem with was when I switched to 2.4 I had an amplified Y harness in my kingcat, it didn't like that with JR8611 servo's. I swapped out the Y harness and used 2 channels instead and never had a problem. I've used non amplified Y harness's in other jets including the flaps on my Firebird and never had an issue there either.