If you prefer to run evaluation on a different validation dataset as opposed to a split from the training dataset, then please convert this dataset into tfrecords as well using the tlt-dataset-convert tool as mentioned here, and use the validation_data_source field in the dataset_config to define this. In this case, please do not forget to remove the validation_fold field from the spec. When generating the TFRecords for evaluation by using the validation_data_source field, please review the notes here.
If you set separate validation_data_source in your training spec, yes, no matter how much you set for the val_split when you generate training tfrecord, the training will load all of your training tfrecords. During validation, the validation will load all the tfrecords in your validation_data_source.