The closest thing to canon within the 'verse is The Verse In Numbers (warning: 7.0M pdf - you might want to click and download that instead of trying to open it up in a browser) which was developed in tandem with the Complete and Official Map of the Verse and is copyright Universal Studios - its about as close as one can get.
The first planets and moons to be terraformed were done so in 2220 (Section 7.17, page 346). The timeline (Section 1.04, page 14) shows this as "robot terraformers arrive and 'tweak' P/2027(White Sun)03 ...)) starting in 2200. So, it took 20 years for the robot Terraformers before the terraforming was complete. It wasn't until 2220 that the ark ships (launched in 2100) arrived and it took 5 years for the colonization of the initial planets to be done.
These robot terraformers were launched as part of a first wave of ships in 2072 (note several years before the airs were launched).
Note that many of the terraforming targets were moons which were then modified with additional gravity screens to adjust the surface gravity.
The technology was such that it was possible to terraform Mars, Venus, and Earth's moon (Section 1.02, page 12; 2037 - 2048).
So, the question then is 'what does tweaked' mean? Its the only place thats used - so those planets may have been almost earth like before the terraforming began. Further information is covered in Section 7.02 (Page 331) "To Terraform or Not" which talks about lifeless rocks, introduction of plant and animal life, and atmospheric processors. Only Londinium and Sihnon are mentioned as 'tweaked'.
Given the wording of the terraforming section, I doubt much of it was habitable before hand. In some cases they ignited brown dwarves to create a habitable zone of what were formerly moons (See Santo, Section 2.23, age 44). Asteroid/dwarf planets were also terraformed (the example given for a minimum size is 970 km - about the size of Ceres).
At this point all one can really do is suggest that there wasn't much life, if any on all but two worlds. Much of the terraforming was done on worlds that would not support life as we know it with the addition of significant technology.