do you use the normal space marine codex's then? And how easy do u think it is if any of you have tried it?
You can use whichever Codex you want, as long as it fits with your playing style. All a Codex really is, when you strip it down, is a set of rules that enable you to build a certain style of army to play.
At the moment I'm putting together a Blood Angels successor army, the Knights of Blood. They are listed in the BA codex and the CSM dex (cool!) so the army I'm putting together is designed to be used with either dex.
I chose the Knights of Blood for a couple of reasons, I really like their colour scheme and there is almost no fluff written about them at all, freeing me up to make them mine and unique. The same could be applied to any chapter you want using any codex you like the look of.
At the more extreme end of the scale, depending on how much work you wanted to put into it you could use a codex for a completely different army. Some people will moan but tough!
You could, for example, use the Necron dex for an Imperial Guard army, you would have to work bloody hard to explain why and how everything works but it could be done. Ultimately what dex and minis you use is entirely down to you, the dex is only rules and the minis are only playing pieces.