Mixing servos is fine. Just make sure they are plugged into the receiver correctly, black/red/other in the correct order. Well, some aren't exactly that, Spektrum servos for example, but the darkest hue for black.

Not the issue it used to be, plugs are standardized now. Back in the day you sometimes had to change the order of the wires in the connectors with some combos. Now it's just making sure the plug isn't in backwards.