Having a mix of analog and digital servos does not matter. My only suggestions is to check very carefully your EPA, even drop it down a few % just to be on the safe side. Also as said before vibration is a big killer. You mentioned that you use the grommets. This is a good thing. I will suggest that you use high end servo's. I have had a few servo's go bad in my buggies. One I pulled apart and looked at the inside of it. I found one of the circuit board tracks was cracked. This is from vibration. I have found that the lower spec servo's are less resistant to vibration.