All I'd say is 1GB sounds a bit on the small side - might cause you some problems in the future (thumbnails folder could end up filling that on its own), and it'll almost certainly cause issues with updates and / or backups not having enough space at some point.
I use small (eg 1GB) partitions for testing, but I'd want at least 4GB (my arbitrary limit) for a 'proper' install.
There shouldn't be an issue with mixing your media (films / music) with libreelec data in 1 big usb partition - the partition even comes with folders for exactly that by default (made on first boot).
Copy that. Thanks, nice to know.
I can easily resize the "disk" partition since the other one is still empty.