For the first question:
Run print(model), you should see the first layer is (conv1): Conv2d(3, 64, kernel_size=(7, 7), stride=(2, 2), padding=(3, 3), bias=False). You modify your first layer without padding and stride, that why it became 26.
For the second question:
Because this is CNN models, so the number of in_channels will not affect to your output shape, the number of class do that.
For example: with ImageNet, the output of ResNet18 is 1x1000,  with CIFAR10, it is 1x10