Im not so sure about the guppy, it is a little temprimental of water conditions, not as hardy as some others. I have lost quite a few males and the odd female when i buy new ones and it always seems to be the prettiest ones that go.
How about Danios, i have never had any so i cant comment, but i have been told they are hardy and pretty good for cycling if you are not doing fishless cycling

Aimee