Sorry about my first post, thought you were looking for emotional support not technical.
Others have chimed in and I agree with them, here's my couple of cents;
Yes, you need the filter gasket. As for the pan gasket, I have never used anything but a plain old pan gasket with no RTV or whatever goo. Just make sure that the mounting flange of the pan is straight and do not over tighten the pan nuts. Loctite is good to use on the bolts here.
As for your leak, maybe the pan, maybe not. Can you see where the leak is coming from?
Potential leaks from:
Pan
input seal (where Torque converter slides into the pump)
driveshaft seal
speedometer o-ring
kickdown cable o-ring
dipstick o-ring
modulator
all of these except the input seal can be replaced easily with the trans in place. Cheap too!
Your new trans fluid will be fine, just filter it through a paint strainer before you put it back in.