E.g. are the clouds on Venus (and Titan) cumulus, stratus, cirrus, or some other type that isn't found on Earth?
Clouds on Venus
After a quick search, it appears the clouds found on Venus are different than Earth's:
Venus is covered by a thick layer of clouds. These clouds are made mostly of sulfuric acid and are very fast moving, going up to 220 miles per hour (350 km per hour). The clouds rotate 60 times faster than the planet rotates. The clouds circle Venus in four Earth days; the planet rotates around its axis in 243 Earth days. These quickly-moving clouds distribute the heat around the planet, making the night side hot also. V-shaped cloud patterns are visible in the middle latitudes when the clouds are examined in ultraviolet light. The clouds rain sulphuric acid (extremely acid rain), but this corrosive precipitation does not reach the surface. The high heat beneath the clouds (up to 220°C) evaporates the rain drops about 10 miles (30 km) above Venus. SOURCE
Clouds on Titan
According to this release by NASA, Titan has "cirrus-like" clouds:
Now, thin, wispy clouds of ice particles, similar to Earth's cirrus clouds, are being reported by Carrie Anderson and Robert Samuelson at NASA's Goddard Space Flight Center in Greenbelt, Md. The findings, published February 1 in Icarus, were made using the Composite Infrared Spectrometer (CIRS) on NASA's Cassini spacecraft.