1. Don't eat it. TRUE This is a big duh but actually this rule sums up how safe it is. You can do just about anything to it, with it but just don't eat it. It's not toxic but it's still not good to eat.
2.. You NEED a dedicated oven. FALSE. The good news is it is safe to use your own oven based on this study from Duke University. You can use a dedicated oven if you want but if you want to spare the expense, your home oven is fine to use. An oven thermometer is a must have have but I discovered my new oven with digital thermometer is super accurate.
If the fumes bother you, some people might not even notice, you can use a covered dish or I discovered plastic turkey bags work really well to keep the fumes in. The fumes are non-toxic unless you burn the clay and this won't happen until 300F for most clays.
3. Any tools or trays you use must be dedicated to clay only TRUE. This is definitely true. Kitchen tools work well for polymer clay - like garlic presses, rolling pins, graters and more. If you use it for clay, make it clay only. You might be able to fully clean nonporous smooth tools but don't tempt fate here. If it is porous in anyway like wood, it's all clay or not at all.
4. Don't use it to make dishes or anything to eat off of. TRUE, polymer clay is porous and even if you glaze it, you can't keep the bacteria out and it is not safe for eating off of either.
5. Don't burn it. TRUE This is crucial, this is the only time the fumes are toxic. Different clays bake at different temperatures so check the package. Fimo will bake at 260 F, Kato needs to be 300F to fully cure. Things can get tricky, I've burnt it with a heat gun and baked it at 400 (for 5 minutes)
The flip side is you can bake it over and over as needed without burning the clay.
6. Don't mix clays. IT DEPENDS As far as baking goes, if you mixing clays - either mixing colors or using different types in one piece there shouldn't be any problems. Most clays cure near 275F so mixing them isn't an issue. I just wouldn't use clays at the 2 extremes (Fimo 230 and Kato at 300) Either one can be mixed with Premo, Sculpey, Cernit etc that cure at 275. I usually use a temperature in the middle to bake it at.
Another consideration is if you are caning. When caning you want all the clays to be the same firmness so when you reduce the cane it will reduce equally. Clays vary firmness by brand and also color to make things more complicated.
7. Beware of phthalates in clay. NA Dioctyl phthalate was used as a plasticizer in polymer clay but due to possible health risks, it was banned. And all clays were reformulated without phthalates. It has made a few clays, like Kato and Fimo Classic, slightly more pliable but they are stiff stiff.
These are just a few of tips and hints about polymer clay clarified. I've also found there were no absolutes. Cynthia Tinapple of PolymerClayDaily has projects when she uses temperatures of 400F (for 5 minutes) and others were she just uses a heat gun.
I personally used my own oven for years until I went to clay guilds and was later told this was a big no-no and I need to buy an dedicated oven. I decided to risk it since I had been using my own for so long; I was glad when I found studies proved I wasn't poisoning my family all along.
There are a lot of tips and advice concerning polymer clay, more than I can list here and many can be broken.