I made a new chapter based on a mixture of space marines and imperial guard called the shijan, who are a secret orginization based on speed and stealth, the problem is i always have trouble with making the rules. Any help, or advice is appreciated!
Basically you don't need to make up own rules for them, as such a combination already exists:
You can play them as an IG army with either grey knights or deathwatch as allies (or as GK with IG allies).
While making your own Chapter is a lot of fun, it usually ends up as a futile effort. You'll only be able to play them among friends, if they allow it, since the rules aren't legal. I rarely see any home grown rules that run out balanced either. You could still do as Archon Astaroth suggested and use an existing set of rules, but develop your own background and history, and paint them to represent your own chapter. That way you still get the satisfaction of building your own army, but they can be played anywhere.
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