Can I install additional software on TLXOS?

Yes.  TLXOS is fundamentally just Debian Linux, so you can use standard Debian tools such as apt-cache and apt-get to locate and install additional software, and perform whatever configuration changes you wish.

Please note, however, that all changes that you make at runtime in TLXOS are initially stored in a ramdisk (the tmpfs top AUFS layer, /run/overlay) and are only committed to permanent storage (the ext3 middle AUFS layer, /config) when the device is shut down or rebooted.  This means that (a) your root filesystem free space is by default limited to 30% of the amount of RAM that is installed, and has an upper limit of 100% of your RAM size (unless you add a swap device), and (b) you should reboot your device after making any changes that you want to keep.

Before you install something large, you should increase the size of /run to make sure that you don't unexpectedly run out of space in the root filesystem, using a command such as:

mount -o remount,size=90% /run

TLXOS has a "User Defined" mode that you can use to launch an application of your choosing on boot.  To do this, edit the /usr/local/bin/userdef script.  Whatever you specify in TMS/Tlxconfig's "Server Name[:Port]" field will be passed as the first parameter to this script, followed by whatever you specify in "Command Line Args".  The userdef script will also have access to environment variables that contain auto login username and password, if these were provided.