The penguin 100 will probably struggle to keep a 20g or larger stable. The gph
rating is said to be obtained without filter media or any of the blockages that naturally develop (mulm, dumb fish that swim up the intake).
If you're worried about stable temps, I'd go with a Marineland Stealth heater a.k
.a visi-therm heater - two, if you're aiming to have redundancy. Cheaper heaters will stay the course for a few months, but ultimately the heating elements tend to fry. The Stealth heaters might be more expensive, but also seem to be more reliable.
All that having been said, redundancy is a bit dangerous - you might not know when one part of your system has failed, overly stressing the backup. You also have more potential points of failure.