HI,
Yes, you are right about capacitance with small cables. So, don't use small cables. And this WILL be a function of cable length. And you are also right about circuits that are not true balanced designs. So, use pre amps that are properly balanced. I know this is a problem, one of many, with inexpensive preamps and recorders. Good balanced circuits are expensive. The pro's use nothing else for a reason.
So, if you must live with unbalanced interfaces, you must also live with the potentially noisy consequences. Try to keep your cable lengths short, route them as far away from AC power wiring as you can, and use quality mic cable. This is not black magic.
Sparky