Welcome to The One Ring!
As to the paints, I'm afraid that I don't use a whole lot of Citadel; but what you're doing to create the stone effect sounds good.
I've got a Minas Tirith army I've been tinkering on for about a year, and I've found that the plastic from your blister packaging can be cut to make excellent stone flagging. If you get a bit of Elmer's (a generic white glue) or a really slow-to-cure modeling glue, you can also spread some earth around on the base to give it more of a feel of being on ruins. Elmer's will make it look like normal ground, while the slow glue will let most of the dirt soak in thoroughly and give you a nice mud effect.
