2.5 gallon for a month

I’m currently moving homes and realizing that I am not a fan of my 5 gallon tank; I’d like to reshape it using the Walstad method, however I know this method will take time to yield results, and in not a fan of the idea of keeping my fish in a cup for a month.

While setting up to buy a plastic bin to keep my fish in for a month; I was recently given the opportunity of having a 2.5 g aquarium with a filter that I really could use as a quarantine tank in the future.

Would it be ill-advised of me to keep my Betta in a 2.5 gallon quarantine tank for a couple of weeks or would it serve me better to make a temporary 5 gallon for my Betta? I’d ideally set up the tank with the same RO water I use, add some plants in cups, and add his old filter media into the tank to keep all the beneficial bacteria and to allow the new filter (sponge) to siphon up all that good stuff.