Can we cross-validate the neuron no. and hidden layer no. of a feed-forward neural network?
Yes. validation is the best way to choose model hyperparameters such as neurons number.
Validation with neural net is not substantial different from validation with other models.
The metric should be chosen according to the task the network performs. The validation method should be chosen according to the amount of data, training time, certainty level you want to achieve and compute and time resources you have.