Mixing Bantams with other chicks?

I'm a new chicken owner, and a newbie here. I'll give some backround info. before I ask my question.

I have 3 3-week-old chicks. 2 are white Leghorns, and the other is a RIR.

Next in line are the 3 chicks that are about 2-weeks-old. 2 are RIRs. The other is a Black Sex Link.

The smallest are 2 little Bantams. They are 3 days old.
Their names are as follows:
Kasie- 3-week-old RIR. She seems to be at the top of the pecking order. Every new chick is pecked once by here before it is allowed by her to be with the others. Kasie doesn't hurt any of the babies, but she does maintain authority.
Anna & Fuzzworth (Fuzzy)- 3-week-old White leghorns. Anna has one dark spot of yellow on her forehead, and Fuzzy has two.
Lily- 2-week-old Black Sex Link. (Where did this breed name orginate?) She is the smallest of the six oldest chicks.
Sarah & Nikki- 2-week-old RIRs. Sarah is very darkly colored and a day or two older. Nikki is very lightly colored.
Amanda- 3-day-old Bantam. The woman at the store said they were not specific chicks, but I read online that Bantams come in may differents breeds. Amanda has fuzzy feet and one black spot on her head. She is brown.
Kelly- 3-day-old Bantam. She is light brown with a dark arrow on her head and down her back.

Kasie, Anna, Fuzzy, Sarah, Lily, and Nikki live in my dog's old cage until they are old enough to go outside.
Kelly and Amanda live in a pet travel crate.

When I got the Bantams, the woman who sold them to us said that our big chickens would peck them to death. Consequently, I have separated them.

Now, for my question. Today I noticed several minutes after putting Kelly and Amanda away, I went to check on them. I found Kelly in the other cage with the bigger chicks. To do this, she had to squeeze out the tiny Pet Taxi bars and walk to the other side of my room, then squeeze into the other cage. I found it odd, but put her back in her own cage. The next time I checked, she had done it again. So for about 45 minutes (I checked in about every 5) I allowed her and Amanda to stay with the older chicks. I did not notice any signs that looked aggressive on the part of the big chicks. I did, however, note that Amanda has taken a fascination in pecking the Leghorns. I just put the younger two back in their cage for the night, draping a towel over them. Hopefully this will disourage them from leaving their cage when I cannot monitor them.

What shall I do? The Bantams really want the company of the older chicks. I don't think my chicks would hurt each other, but other people seem to think so. I'm not sure how to keep the Bantams from leaving their cage even if I have to. I want them to be happy, too. Advice?

 

Hmm....I kind of wonder if that lady knew what she was talking about. In the morning I'll talk to my mom and see if we can move Amanda and Kelly in with th others.