Yes, I drain and refill the coolant on all my cars - I generally stick with the OEM type of fluid. Cost is comparable to others on the market and I know that the OEM fluid will not harm any seals or cause corrosion.
The OEM plug on the Camry is a twin ground plug - reason for it is the system uses a 'waste spark' mode of ignition. This means a single coil pack will fire two sparkplugs simultaneously. That is the reason for the twin electrodes - twice the amount of material that is available to wear down.
Iridium plugs can be used as a replacement - but generally will not last as long as their reported lifespan (100K-120K) because they have twice the normal duty cycle. Their life span will be comparable to the OEM twin plug (about 60K miles) or even slightly less. Will not hurt the motor - iridium is just made from a more durable mateial than a comparable platinum plug.
Advice - stick with the same plug if you are worried or have any doubts about any possible damage to the motor.
2002 Corolla S, Silverstream, 1.8L 1ZZ-FE (VVT-i)
2003 Matrix XRS, Cosmic Blue, 1.8L 2ZZ-GE (VVTL-i) - RIP
2009 Matrix XRS, Blue Streak, 2.4L 2AZ-FE (VVT-i)
