Can I install SQL Server 2012 on a machine with 2008 R2 already?

You can simply install a new, named instance, and the two instances can live side-by-side with no risk - just be sure that your connections strings etc. are clear and correct (many problems arise from connecting to LOCALHOST when you meant to connect to LOCALHOST\OtherInstance).

Depending on resources you may want to be sure to cap both servers using the max server memory configuration option, or perhaps even only running one instance at a time. I assume this is on your local development environment.

I'd suggest abandoning the 2008 R2 version of the client tools and using the 2012 version consistently. It has much better functionality and can manage both versions seamlessly.

Generally yes. Your services on 2008 R2 will continue running normally unless you manually make some changes.

Only impact that sql 2012 can have is if you plan to run some performance intensive operations but nothing else apart from that.

Here is a good article on MSDN on site-by-side support for different versions of SQL Server http://msdn.microsoft.com/en-us/library/ms143694.aspx