Autostart all LightDM seats and show one as default

Autostart all LightDM seats and show one as default

I configured a media-center server running Debian with LightDM, Leapcast
and Plex. What I want to accomplish is the following. There should be
three LightDM seats, one default seat on which I can login; one seat that
runs Leapcast (Chromecast emulator); and one seat that runs Plex Media
Center. The last seat should show up as default.
Below you can find the relevant part of my lightdm.conf:
[Seat:0]
vt=7
[Seat:1]
allow-guests=false
autologin-user=media-center
autologin-timeout=0
greeter-hide-users=true
session-setup-script=start-chromecast
vt=8
[Seat:2]
allow-guests=false
autologin-user=media-center
autologin-timeout=0
greeter-hide-users=true
session-setup-script=start-media-center
vt=9
This configuration file enables the three seats, as intended. The problem
I am facing now is that the default seat (the seat that is shown after
booting) seems to be random, sometimes tty7 show up at boot time (showing
the login screen) and sometimes tty8 shows up (which auto-logins and runs
Leapcast). Another problem is that when tty7 shows up as default, tty8 is
not started automatically.
So what I want is
to be able to choose the default virtual terminal and
make virtual terminal 8 and 9 (Leapcast and Plex) start automatically.
Thanks!