Good work! 
I especially like "The Certified".
They have a pretty imaginative and different background that you could build on.
Cheers mate, glad you liked it!
Not entirely certain about the colour scheme, but the background is cool! 
I’ll redo it when the new B&C painter is online, I thinking of making it more obviously a cross on their armour.
Love your artwork to by the way, and especially the Tau redesigns which I think look better than the original.
Cause a bit of controversy among the hardcore Tau fans, but on the whole I think the liked the attempt.
I'm also not saying that Philip S. should abandon his Aztec take on the Chapter, just adding as further commentary my own take on the Rainbow Warriors. A destroyed Chapter (whether in actual fluff or otherwise) doesn't mean that someone shouldn't flesh them out.
I wanted to work up the rainbow warriors, because most think it is named after a ship, though there are many rainbow warrior myths (native American) to draw inspiration from, and the rainbow bridge Norse mythology.
I not saying it wasn’t named after the ship mind! But I think it was something that I could work with, and blend into the 40K ethos other flipping preconceived ideas.
I really like the 'Certified'. Honestly, they've got an intriguing and interesting basis for the background and I'd love to see them expanded. I'd love to do the expanding!
Go ahead and play with it. I wrote them to inspire others, and if you feel inspired: get cracking!
With regards to the Space Sharks, however, I like the version put forth in Cpt. Tiberius' thread; an interesting idea and gorgeous artwork, too! 
I do like the artwork.
For my Space Sharks I wanted something that wasn’t based around the sea on their home world, as I felt this would be a lost opportunity. As they are
Space Sharks, I imagined the sea of these sharks to be
space.
And all the hunting tactics of the sharks kinda fell into place.
Like all these ideas, it's the core concept I after at the moment. Once the concept is fully crystallised, the background is easy!
Philip
PS: Sons of Sparta with change, I wanted to introduce a St George figure, but he was not born in Sparta! (I used this not because of the movie, but because of the Golden Daemon entry from yesteryear). I’ll make up a Turkish (or Turkish myth) based chapter and reference that instead. This was to point out that St George may be the patron saint of England but he’s not English! Thus Sgt (sergeant) Georgios was born