2.5 g would be great for killifish or for maybe a shrimp group for feeders...
5g. would be good for a quarantine or for a holding mom to be able to spit on her own...
10 gal would be good for a pair of Kribs, or shellies, or a small group of fry for a few months...
You could use a small tank to grow anubias emersed (I hear they grow a leaf per week this way!) LFS's usually like these...
I would definitely take them! :thumb: Lucky you!