Baking temps can be quite problematic, especially these days since the new formulations of the Fimos have come out with their new recommended temp of 230 instead of 265. Other clayers have found that it's okay to use the old temp though instead of the new one, especially if one is baking "carefully" (with an oven thermometer, not close to the coils or sides, on a good baking surface, etc.).
However, there is a problem with the Sculpeys* even at their recommended temp (275) since they tend to darken very easily, compared to other brands baked at their recommended temps... Polyform doesn't really address this issue because they know the higher temp is really required for strength and they think that "crafters want to bake quickly."
Using a partly-"enclosed" or a fully-enclosed baking method can keep even any of the Sulpeys from darkening though (one of which is to drape a damp paper towel over the object while curing, as mentioned above).
"the Sculpeys" are mostly Sculpey, SuperSculpey, and Sculpey III -- Premo isn't a "Sculpey" to most clayers because it is a quite different clay, and was created to the specifications of a clayer with the Polyform chemist simply carrying out her wishes
Baking times are also a bit weird.
It used to be that all the polymer clays gave the 15-20 minutes per quarter-inch recommendation, but for much thicker items it really doesn't take as long as the math would suggest. On the other hand, any polymer clay will get stronger than it normally would have with longer baking because the polymerization will continue and continue. (Time and temp. are interdepent for thermosetting plastics like polymer clay, so the lower the temp the longer the baking required to achieve sufficient polymerization--there's even an equation for that relationship).
As for mixing clays, the general advice was always to decide on the temp based on the relative
amounts of each brand used in a particular mix. But it's actually better to bake them all at say 265-275 for as long as is possible without causing the mix to darken
(which can be helped with the techniques mentioned above). If you bake at a lower temp (230, e.g.) then you'll certainly need to bake non-Fimo clays longer to get a good amount of polymerization --which btw isn't just when the clay feels
hard, because the outer portions of the clay will polymerize sooner than the interior will so you wouldn't have any way to know
that the item hadn't polymerized well all the way to the center.
If you want much
more info on all aspects of baking polymer clay, check out this page:http://glassattic.com/polymer/baking.htm