My advice would be don't do it. I don't think you can mix a musical properly, i.e line by line, on 2 inch long faders controlled with a mouse.
However using QLab to control a digital mixing desk by MIDI cues or OSC, is practical.
OR
If you only had access to a basic analog desk with no automation, you could come out of the insert send ON EACH CHANNEL into an input on your interface into Qlab and then back out of the interface to the insert return.
That way you can use the faders and EQ on the analog desk to mix, and QLab Mic cues to do mute automation and some special EQs and effects automation.
If you really want to try and do it all in Qlab, then Vincent's No2 scheme is probably best, as otherwise if you have all the mics set in a cue for a scene, any Audio Effect Plugin you use is going to be universal to all channels.
You seem to be suggesting you are going to use 2 interface devices to get your input count. You can only do this using an aggregate device. Before you do this I would do a search of this google group for 'aggregate device' and take note of all the advice given in these posts.
If you use Qlab on analog desk inserts then you don't need to aggregate the devices as the input and output for each analog insert will be on the same device anyway.
And finally, whichever method you choose, if you are not doing too much playback of effects, and your use of plug-ins is modest, you may be able to reduce the buffer sizes in preferences to reduce latency.
Good Luck!
Mic