Best food-getting new gerbils

It's been a while since I had gerbils. So, I'm a little nervous. But long story short I'm rescuing 7 gerbils from a "feeder breeder." I'm not planning on keeping them long, just fostering until they find their forever home, I would love to but I already have TONS of animals.

But I was hoping someone could help me out with their food. What is the best food available for them? I want to make sure they are getting fed good high quality food. Right not they are getting fed Mazuri, which I know it's a bad food, but it's also not the best.

Also I feed my hamsters and mice "Native Earth 18% Protein 4018 Rodent Diet" (http://www.petfooddirect.com/product/9093/Native-Earth-18-Percent-Protein-4018-Rodent-Diet-40-lbs). Would that be ok to feed gerbils. Just as a back up, not as a staple. Because I know they need a variety.

Also any more info would be great. I know no ceder or pine. When I had gerbils I would shred cardboard and use that as bedding and they LOVED it, I would add more boxes all the time. By the time I had to clean their cage out the bedding went half way up the cage, which they LOVED. The LOVED digging it it.

Sorry this is so long. I just want to make sure they are taken care of properly.

Thanks for any help!!!