there are no longer any special rules concerning catachan and cadian, now it's all guard. If you want some sort of specialist identity for your Catachans make them Veteran Guardsmen and take your cadians from the platoon list.
As for tips playing guard, depends on what strikes your interest. In earlier editions I played infantry heavy as tanks were so easily detroyed and I didn't have the funds to purchase 8 tanks in addition to all my guardsmen. However now tanks come in groups and cover saves provide a little more survivability than they use to. Really what interests you the most about Guard?