yes that will work. It does take care of out of sync cards by using SRC to correct. However, I would rather use the zita-ajbridge package which has zita-a2j instead of alsa_in and zita-j2a instead of alsa_out. The advantage with zita-ajbridge is that is has better quality SRC while at the same time using less cpu power. It also allows not using the SRC if you have a word clock link between your devices and they are synced. Another note is that jackd 1 has this built in already though jackd 2 does not yet.
Any time SRC is used, there is no longer a sample for sample path. The difference for most cases is not audible but I would not use sources across two device for stereo or other surround tricks. Left and Right for any source should always be on the same device. A second left and right can be from another device.
SRC = Sample Rate Conversion
MIDI is a different animal. You can use as many MIDI devices as you like and jack with a2jmidid will show all of them with no problems. It is even possible to use a midi port on a card and not use the audio part. for example I have an old pciaudio card that has a joystick/midi connector that I use only for MIDI no audio.