welcome to the forum.
1. The original 1960's Thingmaker has a pocket in it that fits the 60's molds. The 60's molds are smaller than the Toymax and Jakks molds, so no the new ones don't fit. The newer ovens with the light bulb do fit all size molds but they take longer to heat your mold. Personally I use a toaster oven. With the aluminum tray I can cook 6 at a time so me and the kids can all do them together. Just set the temp for around 300 F and adjust from there. I had a friend that used to use an electric waffle iron.
2. There are all kinds of goop out there. One thing to consider is the use of phthalates which is banned in the US and many other countries from being used in kids toys. Some amateurs out there buy goop with phthalates that are not meant to be used by children. They then bottle it themselves and sell it to unknowing parents anyways. The old original formula of plastigoop from the 1960's had phthalates. Since 1980 official plastigoop from Toymax and Jakks does not have phthalates. Dr. Goop and Plasti-Craft brand goop does not contain phthalates. There is no compromise using the modern goop, it works exactly the same without the health hazard.
3. There are paints out there and I did experiment with some that work great on Creepy Crawlers. Unfortunately they use hazardous solvents that kids should never come in contact with. Instead try the method of adding the color goop to just that part of the mold you want colored. Then bake it. After it cools you can add the next color. By baking each color it gels and this prevents it from running into the other colors. Let me know if you need more detail on how to do that.
4. Tips on baking: The time can vary a lot when cooking up Creepy Crawlers. It depends on how hot your Thingmaker gets and how big of a mold and how much plastigoop you are putting in. For me the best way to tell is by watching for the goop to go from a cloudy to a more solid color. Once you try it once and watch you will see and be able to tell when the goop is fulled melted. Another good indicator is when the smoke starts. Now if your Creepies pass the rip test then they are full gelled (cooked). Only try the rip test on room temp Creepy Crawlers since the hot ones will rip much easier. So once they are at room temp see if they stretch and flex like rubber, without breaking. If they are not cooked all the way you might put some fresh goop on the rip surfaces, put it back in the mold, and re-cook it. Also check out http://dr-goop.com/info
Hope that helps. Let us know if you have any more questions,