Can I ride a Tokyo city bus with SUICA/PASMO?

Yes. The price is 210 yen for an adult if you pay either by card or cash, 100 yen by cash or 103 yen by SUICA/PASMO for a child (6 to 11 years old, but 12-years old elementary school childrean are included). Up to two children of 1-5 years old per adult are free. Children under 1 years old is always free.
Get on from the front door. Pay to the driver by your IC Card, coins, or a 1,000 yen bank note. 2000/5000/10000 yen notes are not accepted.
When you want to get off at the next stop, press a button near you.
Get off from the automatic door at the middle of the bus.
Do not stand too close the exit door, because the automatic safety system will not allow the door to open, when someone is standing too close to the door.

How much is the fixed taxi fare from airports to Central Tokyo?

Some taxi companies offer fixed prices from airports based on area and time.

From Narita Airport, fixed prices of taxis ranges between about 16,000 - 24,500 yen through ordinary roads in the daytime.
If you want to go through highways, the prices ranges 18,500 - 26,500 in the daytime, and you will be asked to pay additional cost of highways.
For details, see their official website:
  Taxi (Narita Airport Official Site)

From Haneda about 5,900 - 11,200 in daytime, and 7,100 - 13,200 at late night (22:00 - 05:00).
For details, see their official website:
  Taxi (Tokyo Hire-Taxi Association