Yes, if you want to use btrfs for / you will have to make an ext4 partition for /boot before installing, as you know lilo can't cope with a btrfs file system for /boot.
But you shouldn't need to use
gparted for that (though that's another possibility).
For /boot, 20 M should suffice unless you want to add a custom kernel later and 30 M should be more than enough in that case.
To partition the hard disk, just launch the installer, and as soon as logged in as root (but
before running setup), type either
cfdisk or
fdisk as you prefer. I'm assuming that you are installing on a machine with a BIOS firmware, not UEFI (in the latter case you should use gdisk or cgdisk instead and make an /EFI partition in addition).
Let's take an example. I assume that you'll have created a / partition called /dev/sda1, a /boot partition called /dev/sda2 (both of type Linux, code 82), a swap partition (Linux swap, code 83) and maybe others of type Linux.
When you are done, type
setup. First, choose SWAP to set up yous swap partition, then use the TARGET entry menu to format the other partitions. Choose to format them with file system btrfs for / (dev/sda1 in the example), then ext4 for /boot (/dev/sda2 in the example), others if you want (though useless in my humble opinion).
While formatting them the installer will automatically mount the / and /boot partitions to allow you to install the packages in the INSTALL step, so you don't have to do that yourself.
Then in SOURCE choose the source of packages you want.This just can't be the Slint media as it doesn't contain the Slackware packages (yet). At this step you can safely remove that Slint media if need be to replace it by another media, as the installer will have been already loaded in RAM (in fact that's done as soon as the installer has booted).
Then you'll have a chance to INSTALL all what you want and only what you want. If you need an advice for the choices of packages to install, just ask.
Oh, and of course I'm willing to help you installing Slackware
Good luck, let us know how that goes.