Sorry tried to post a couple of hours ago, but lost our internet connection. As was mentioned, if the chicks have progressed to this point, size differential shouldn't be a problem. We've had a number of "runts" that came along after the others and they turned out fine.
As for the mutations, technically, a Zebra with even the slightest little white spot is considered pied. Too, if you don't know the heritage of your adults, it's hard to say what recessive splits they could be carrying. We've had couples wherein one of the adults were Fawn, White or Pied and they've produced at least some Normal Gray. In turn when those Normal Grays went on to produce offspring a number of them ended up resembling Grandma and/or Grandpa. Zebras can even carry splits generations beyond that as well.
I would have to agree that your two chicks are Fawn, Pied, or some sort of White. The link Domenic gave you is an excellent reference to help sort out the various intricacies of Zebra genetics.
One additional thought. There is at least one mutation that can be deceptive. There are birds that appear to be Normal Grays that really are Single Factor Florida Fancy. We had a breeding couple where the male was mostly white and the hen was of a fawnish color and all told they produced four males that looked like they were Normal Gray. Turned out when some of them reproduced they had chicks that had a base coloring like their mother, only being males, they had full markings. Our investigation led us to discover that not only were the young birds Single Factor Florida Fancy, their dads were as well. These pics are a good representation of first the dads and their subsequent offspring.
http://www.efinch.com/birdpix4/sf_df_ff.jpg
http://www.efinch.com/birdpix4/sfff.jpg
Notice the lack of a pronounced breast bar in the darker male in the first pic. A tell tale sign of the SF/FF. There are other indicators as well, but I'm not well versed in enough Zebra genetics to point them out.
Enjoy the antics of your babies, you will be greatly entertained over the next month or so. And remember, more pics are always welcome.
Rory and Sherry
30+ Zebra Finches (The actual number constantly changes) 2 Rabbits (1 Mini Rex, 1 Dutch), 1 Grouchy Old Fart, and 1 Dang Redhead
I am going to guess that both of your parents are pied, and that you have 2 babies that are going to be either all or mostly white or cream, congrats on a great looking clutch. 
Michelle
