"using wheel weights, can you use straight ww's to cast bullets for guns such as the 30.06 and 9mm?"
Sure, you can use it. But...it really needs a bit of tin to cast AND shoot better.
Part of what happens during cooling/solidification of WW metal is the antimony crystalizes out before the lead solidifies. The (hard) antimony is then held in the lead as a two part matrix, much like rocks in concrete, leaving the exposed soft lead to smear onto a bore more than it should. That does't matter for wheel weights but it does for bullets. A little bit of tin changes the mixture into a real alloy and that makes MUCH better bullets.
"do you need to mix it with another alloy to make it softer?"
Wheel weights don't need to be softer so far as shooting it goes. It does have quite a bit more antimony than is actually needed, especially if you add the tin, so it's something of a waste that could help harden pure lead if you have some of that too.
Any of the older Lyman Cast Bullet handbooks are good, as is the NRA published "Cast bullets" by Harrison. It's out of print of course but Amazon's used book sellers usually have it available for reasonable prices. There is little new about bullet casting that the old guys didn't know.