Can I safely mix the standard servos with the digital servos on the same system?
The S3004 servos are single BB digital servos but they have slightly different preformance.
Here are the specs from TH for the S3004:
Speed: 0.23 sec/60 degrees at 4.8V
0.19 sec/60 degrees at 6.0V
Torque: 44 oz-in (3.2kg-cm) at 4.8V
57 oz-in (4.1kg-cm) at 6.0V
Length: 1.6" (41mm)
Width: 0.8" (20mm)
Height: 1.4" (36mm)
Weight: 1.3 ounce (37.2 grams)
Connector: "J" type with approx. 5" wire
And the S3151
Speed: 0.21 sec/60 degrees at 4.8V
Torque: 43.1 oz-in @ 4.8V
Dimensions: 1.59x0.79x1.42 in (40.6x20x36mm)
Weight: 1.48oz (42g)
So as I see it, as long as you use the servos in pairs (same servo for both ailerons or pull-pull), it should be fine.
Hope this helps.