Hi kevin007 
In my experience, sterbai have to be mature to start laying eggs, and they are not particularly noticeable when they are carrying them. So, I don't think you have to worry. Some corys are easier to tell the sexes apart than with others, and sterbai seem to be similar to each other in size. With the first sterbai I got, I was not entirely sure that I had a female until she actually spawned.
In any event, don't get your hopes up too much. Their first spawning is likely to be small anyway, but will indicate to you that she is ready. Then you can spend time conditioning them, and getting a spawning tank ready.
Do not consider putting any small fry in a tank with guppy fry. The guppies grow much faster than the cory fry and will actually attack them at a very young age. I learned this lesson the hard way. 
In my experience, sterbai have to be mature to start laying eggs, and they are not particularly noticeable when they are carrying them. So, I don't think you have to worry. Some corys are easier to tell the sexes apart than with others, and sterbai seem to be similar to each other in size. With the first sterbai I got, I was not entirely sure that I had a female until she actually spawned.
In any event, don't get your hopes up too much. Their first spawning is likely to be small anyway, but will indicate to you that she is ready. Then you can spend time conditioning them, and getting a spawning tank ready.
Do not consider putting any small fry in a tank with guppy fry. The guppies grow much faster than the cory fry and will actually attack them at a very young age.
I learned this lesson the hard way. 
