Forgive me if I have mis-read your question, as a lack of periods certainly make it difficult to understand. I'll do my best though.
Yes you should be able to use an aftermarket temp gauge.
The temp gauge has a sensor inside the engine that it needs to be connected to, but that's NOT the same temp sensor that makes the car run correctly.
I'd be willing to bet your making this more difficult on yourself than it has to be though.
There is a small green wire coming off the (temp gauge) engine temp sensor. This is located on the passenger side cylinder head. This wire gets broken VERY easily when changing spark plugs. If this wire is broken, your temp gauge wont work. Check to see that your current temp gauge is connected before you determine that your current cluster is broken.