Hey, that's a good idea to use the slave soldiers as cheap patrollers
Do the conscripts cost gold upkeep? That would negate the slightly higher tax rates you would get if they do.
The way I play Mictlan is this simple equation:
Gold = Blood Hunters = Blood Slaves = Demon Horde
By adding in the troops you take away from the demon horde result. The only things I use gold for are Mercs, Priests of all flavor, Sages or other Indie Researchers, Scouts, Watch Towers, Temples, Laboratories. Possibly Province Defense if I'm being harrassed by weak raiding forces.
The high mobility of the flying Demon army lead by a Arch Devil is that you can defend your watchtowers easily with it due to their strategic movement. Flying 3 provinces a turn in any direction allows you to hit an army as soon as it enters one of your provinces. You may lose a Watchtower here and there but it's not like they are hard to conquer back.