Hello Kanyanlim

Welcome to FishLore - I hope you're enjoying the forum.
Guppies need about 10g because they need swim room, so they're not a good option for your new tank.
Your 2.5 gallon tank would make a perfect home for an African Dwarf Frog, once the tank is cycled. They are adorable little creatures; here's some info on them:
Care Sheet For African Dwarf Frogs
It's really too small for fishies because there's just not enough room to swim and stretch their fins.

Great for an ADF, though!