IF I STACK 10 HEAVY TANKS AND 5 SP ANTI AIR, WOULD MY ANTI AIR WORKS IF I GOT ATTACKED BY ENEMY PLANES?

YES

yes

Thank you!

Is it better to send 25 tanks and 25 Anti air in one stack? It’s gonna be 50/10

if economically feasible, yes

No, probably not. 50 units is overkill. If you want to protect your damage-dealing units from losing too much health there are cheaper meat shields than tanks and SPAA.