would solder be strong enough for small projects such as customizing scale metal models.....or should i use something else? any information would e helpful...ty

get some paper clips and a torch lighter.

I believe you should do the test as stated above^^^. Also, I know people who make stained glass projects use solder ot hold the glass pieces together. They wrap the glass in copper, then solder the copper together. Hope it helps ;)
Dave

Those toy cars are made of zinc or some alloy. Hard to solder.

Depending on the application, Solder would be fine. But you will not be able to solder to diecast. You can try it. But solder does not adhere to aluminum.
You could use a high quality epoxy to make changes. When it dries you can trim and shape it with a hobby knife and finish sand it.