Is Playdoh safe for making custom gummy bears?

NOOOOO, Playdoh will not work. At all. Vacu-forming could work, but is it really that much cheaper? Like legitimate question, I have no idea but it seems expensive. Look into other Smooth-On products, they might have a food-safe resin (not flexible like silicone, but your mold probably won't be complex enough to need silicone. Don't underestimate it though, you're getting what you pay for. Smooth-On is THE brand. Anything else, make sure you do your research and make SURE anything you use is food safe.

Silicone is food safe. I am fairly certain that Playdoh is not for pouring hot candy material into. And if you think silicone is expensive, wait till you start buying vacuum forming equipment. Shop around for silicone in smaller batches.

Will you be making candy or other things that are liquidy rather than stiffer in your molds or ones that need to be heated rather than air-dried? If so, you'll probably need one of the "pourable" types of silicone molding materials (those are safe for food where many other molding materials aren't necessarily safe for food contact, especially wet food, or they're porous and would have to be sealed and a release used. If you don't need to heat the candy/etc in the mold, the options would be greater but pretty fiddly (e.g., making molds from good-quality air-dry clays --not Play-Doh btw, but perhaps something like a good salt dough clay, etc-- then sealing them with something like polyurethane, then using a good coat of edible cooking oil as a release?).

The mold making material you linked to is one of the "pourable" types, but you could also use latex molding materials that are brush-on instead (many layers, letting set between each layer) which are cheaper.

Or you could use one of the two-part silicone molding "putties" which come in smaller containers. They're expensive by volume (though can buy smaller volumes, like perhaps $20-25) but you wouldn't be wasting a lot of the putty in-between the actual molds. Instead you'd usually just make an individual mold for each item, and use only enough to press into the item. Making 3-D molds rather than "push molds" where the casts made in the push molds will be flat on the back but completely dimensional on the front side is more difficult though, no matter the material. You might be able to make 3-D molds though of small items that didn't have thin projecting areas with the flexible silicones or latex, or make "2-part molds" with most any material as long as neither side had undercuts on its own.

Most of the 2-part silicone putties are about the same prices but quality does vary. The best brand you could probably buy locally (at craft stores, etc) would be Easy Mold (I wouldn't recommend Amazing Mold Putty), or if buying online probably one like Miracle Mold or Alley Goop.

You could probably find lots of sites and videos online for using those in various ways by just looking up those brand names (plus perhaps the words mold making, etc).