You seemingly have this wrong.
The LXDE "750 Mb" .iso is a "Live CD" including a "full" Debian distro using LXDE as desktop/graphical environment, there are a similar one using Fedora as base OS and yet another using Lubuntu (which is 696 Mb instead).
They are "working builds" to demonstrate the use of LXDE.
Normally you would install "a" Linux distro, then install to it LXDE and use it as desktop/graphical environment, replacing the one the "plain distro" uses by default.
From this to re-package this install into a .iso (be it a Live CD or a simpler "install CD") there is a large (very large) leap.
The slackware "mini", as explained here:
If you're planning to install Slackware using packages on a local hard disk
partition, or from a NFS/HTTP/FTP server, you don't need a full-blown CDROM
to boot from.
This mini-ISO image of less than 40MB in size contains everything to let you
do the above, and nothing more than that.
is the bare minimum to start installing (on hard disk and through packages) a "customized" Slackware.
In layman terms, it is like you want to build a new house (without having ever worked in the field) and start by getting just a lot of timber, a hammer, a few nails and a hand saw.
It is of course possible that you manage to build yourself (or adapt what you can find locally) all the pieces needed to have the house built, and assemble them properly, but it will be a loooong process.
When it comes to making a .iso of it, it's more like you disassemble the house you just finished building, package every piece neatly and in order, make drawings and assembling instructions for it, and then give the whole lot to someone else that uses that for re-building the house.
Not something I would advise.