It sounds like they are too big for him. You should use them as a starter colony and breed them. Then you will have all different size roaches to feed.
I got mine from here. He only sells nymphs so I'm slowly building up my colony.
They like it very warm and humid.
http://shop.bugsincyberspace.com/Green-Banana-Roaches-2-dozen-nymphs-bic207.htm
That would be Titan. I believe he's a male. The color is more blue than green but doesnt show up well in the picture.
Heres a picture of Matilda. It's not very good. She's hard to get a picture of because she is shy but very cute!:)