Yap if it's just an inch just add some no need to mix. Just don't dump the whole bottle in their. If you go over it will just go down the overflow tube and be wasted. Celica's get low on radiator fluid it's normal.
If it's time for your coolant flush I'd reccomend doing it. It's a working process. Takes time to flush the Celica's coolant all out and add new coolant but a great learning experience.
Glad to hear it's normal, I was getting worried
What coolant do you recommend I buy?