The Walrus matched British shipboard aircraft requirements

Were American Kingfisher and Seamew floatplanes too large for British cruisers, battlecruisers, and battleships?

Not necessarily, but Britain already had ample Walrus aircraft whose transport and recovery capabilities matched Royal Navy requirements.

The American aircraft could probably have been accommodated, but accommodation was not the decisive requirement. The Walrus already combined observation with transport and recovery work in a form the Royal Navy understood, supplied, and operated in quantity.