You mention that you have trained a facedetect model. Then, may I know if you want to classify the faces into 3 kinds of classes: age, gender, emotions? If yes, for your question 1, please follow the classification model or jupyter notebook to train 3 kinds of TLT classification model. For example, for age classification model, you can prepare faces and split them into different ages. Then put them into different folders and train. After all the 3 classification model are trained, run inference as the secondary engines. See /opt/nvidia/deepstream/deepstream-5.0/samples/configs/deepstream-app/source4_1080p_dec_infer-resnet_tracker_sgie_tiled_display_int8.txt for reference about the config file.
For you question 2, if you train classification model, for your case, actually you just need to prepare the face dataset. If you do not have face dataset, you can search help from websites. For example, GitHub - ageitgey/face_recognition: The world's simplest facial recognition api for Python and the command line will find all the faces that appear in a picture.