My searching skills might suck.

replacing the water pump and housing with new. The old sending unit had a crush washer but haven’t seen if that should be there or not?

it’s a low mileage car so I assumed it was stock. Replacing both the sending unit and fan sensor as well.

is high temp thread sealant sufficient?

 

Washer needed for water temp. The fan switch uses a little sealant but thread tape will be fine. If you're using a non stock ecu, you don't need the fan switch at all.

 

Fan switch O-Ring: F8B5-18-T41 (If you get a new sensor then it'll come with a new O-Ring)
Water Temp Sensor Washer: 9956-41-200

No thread sealant is needed for these.

 

9956-41-200 decodes like this

99 = Hardware, bolts, nuts, washers, etc
56 = washer
4 = Aluminum, 2 is copper
12 = is the size, 12mm ID, Mazda has numbers from 4-5mm all the way to 21mm
00 = if it was a bolt the length would be here

so 9956-41-200 is an aluminum crush washer with a 12mm ID