Hi,
KiCad is very easy, since I’ve used it several times, and you can select the rules as follows:
Clic “Design Rules” -> “Design Rules”.
In the window type your rules, I usually use 8 mil (0.0080) for the tracks and the margin.
Via Dia: 0.0295
Via Drill: 0.0197
You can use smaller sizes.
Either way, if something is wrong they’re going to tell, so do not worry.
I’ve never had problems, and I’m very happy with the service. Good luck!