can I use the trackpad (2012) on the iMac 2018?

There was no 2018 iMac, there are 2017 & 2019 models. However the answer is yes, there should be no problem.

you can use any trackpad or mouse that support Bluetooth even if its made for windows or Linux computers Bluetooth is generic in terms of drivers

I am using a original Magic Trackpad I bought not long after they came out in 2010 with a 2017 5K 27-inch iMac with zero issues.

Yes.