'Habitable' planets fall into two distinct categories, IMO.
a) The planet has the resources necessary for a permanent, self-sufficient colony to be established. You may not be able to walk outside and breathe the air, but you are not utterly dependent on the monthly supply ship for your air, water, food, and raw materials.
In this sense, almost any planet that has a solid surface and does not have corrosive/insidious conditions, could be habitable - particularly at higher TLs. There might be nothing to prevent arcologies from being established on (or in) almost any world around almost any star, provided that Hydrogen, Oxygen, and certain minerals and metals are present.
b) The planet has a breathable atmosphere and surface conditions in which a TL0 human culture could thrive. This will almost certainly require indigenous and human-compatible life-forms to create a human-friendly environment. These are likely to be much rarer, and probably will not be found around very big or very small stars. They may even be exceedingly rare around sun-like stars.
This type of world could, perhaps, be terraformed from the other type, but even so, would need to have orbital conditions that would not destroy the terraforming over time. Therefore terraformed worlds (whether by humans or Ancients) again, probably would not often be found around very small or very large stars.
However, because we like our sci-fi, and we want to enjoy a good game, we have to suspend a certain amount of disbelief. Depends how 'hard' you like your sci-fi.