You mean take media from the larger tank to add to the 8 gallon? It should be fine. I'd assume maybe 20-30% of a filter cartridge would handle the new tank no problem. Don't be afraid to take a pair of scissors to the filter to get the size you need. If it's filled with carbon you can just shake it into the trash.
*Oh wait! I have it backwards.
I'd still vote the same though. 20-30% removal shouldn't be a problem IMO
...just watch your levels obviously. A piece of media from an 8 gallon won't be a huge help so don't go crazy stocking it...but it'll introduce some bacteria to get things started