How does your group handle the classes in First Founding and successor chapters? Would you allow for an Ultramarine descendant to become an Honor Guard for example. I'm aware in the fluff some of these classes are shared by successors(Blood Angels and their successors both use Sanguinary Priests). How does handle this?

Unless the successor is radically different to their progenitor chapter, I'd allow the classes. And the Honour Guard class is outright stated to have equivalent versions in other chapters (e.g. Blood Angel Sanguinary Guard).

Rather than mix Chapters, our group looked at all the existing advanced classes, and then either:

•  modified an existing one to fit our current Chapters, or
• built entirely new specialities that fit in with our Chapter histories / idealogies etc.

We've tried to make as balanced as possible (with the other advanced classes), and all those that GM also reviewed them, to ensure no 1 Chapter has advantage over the others.

Sounds good, Sentinal666. I do think most chapters aren't sufficiently different from their progenitors to warrant different classes but a disproportionate number of PCs are from more divergent chapters.