Hi hundido, and 
Consider yourself fortunate in that you were confused. No, Really. 
Menu>Setup>puppy installer is a 'grand-fathered' application, hanging around as a lot of newbies discover Puppy when they are trying to keep an older computer out of a landfill. Puppy Installer doesn't work with newer computers which employ UEFI as a boot mechanism.
And, as you've discovered, balena etcher --which it may be able to install a Puppy to a USB-Stick-- doesn't do it properly. The result is a Puppy which won't save your configurations, settings and the applications you want to add.
The application you want to use to create a boot-able USB-Stick is frugalpup, viewtopic.php?p=1588#p1588. gyrog has also published a companion application known as OtherInstalls, viewtopic.php?p=24919#p24919. OtherInstalls enables adding 'non-puppy' operating systems such as Windows [and maybe Fedora*] to a grub2 menu.
[* Let us know if you try to use it with Fedora and how it goes. You may notice that it's still in the Cutting Edge Section of the Forum. But I'm sure gyrog would be interested in your experience with it].
For now, lets stick to trying to get your Fossapup64 and slacko 7 properly running. Fossapup64 has a version of Frugalpup builtin and appearing on the Setup-Submenu. If I'm not mistaken, it's version 20 and frugalpup is now up to version 33. If you have been able to create a SaveFile or Folder I would tell you to download and install the version 33 pet*. It should be possible to install version 33 and Menu>Exit>Restart Graphical Server which causes Puppy to 're-catalog' what applications make up its system, and thus use version 33 without being able to execute a Save. But for the purpose of creating a 'Puppy USB-Key', your builtin version 20 should suffice.
You'll find instructions for using Frugalpup here, http://www.fishprogs.software/puppy/fru ... index.html. Note that frugalpup DOES NO FORMATTING. So use your current USB-Fossapup to do prepare the USB-Stick: Menu>System>gparted.
The easiest way would be to then employ "Stickpup" after reading the instructions. It's part of Frugalpup but (under version 20) has its own menu listing: Menu>Setup>StickPup. But for greater flexibility (such as being able to boot both Fossapup and Slacko 7 from the same USB-Stick, or to merely use the Stick as to boot them and other Puppys from USB-Sticks and hard-drives; and have the advantages of having them boot from and/or have access to Linux Formatted partitions) I recommend that you read thru this thread, viewtopic.php?f=105&t=337 especially bigpup's advice.
Unfortunately, at the moment I can't be more specific other than to tell you that the way I setup Puppys on a USB-Key is to create 3 partitions: (1) the first a small (<500 Mb) fat32 boot partition; (2) the second a fat32 partition large enough to locate files I may later want to transfer to a Windows computer; and the rest Linux Ext3 to hold Puppys, SFSes, AppImages, and portable-applications and engage in any operations which can only be performed in Linux formatted space.
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* The way Frugal Puppys work is that new applications are initially only in RAM until you execute a Save, at which time they are written to your SaveFile/Folder. Applications in a SaveFile/Folder have priority over 'builtin' applications. There is also an SFS version of Frugalpup 33 which would be great if Fossapup64 didn't already have a builtin version. SFSes aren't installed. Essentially, they are 'portable', loaded into RAM when needed, and can be unloaded when not. [Again, a device for old computers with little RAM]. But they have lower priority than either the Applications in a SaveFile/Folder or builtin applications.
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