Nicaea didn't meet to decide the canon. They didn't issue any rulings about it, in fact there's no positive evidence that they even discussed it. This is a common myth about Nicaea and the Canon.

As for groups, this was a council of bishops from the mainstream church, ie. Catholic before there even the East West schism. Although schismatic and heretical groups existed (eg Marcionites, Donatists, etc.), none would have been invited or included at the council.

For the last part of your question, there's an interesting book by Bart Ehrman called The Lost Christianities that you may like.