Add instance to cluster that has been patched

I need to add a instance to an existing SQL 2008R2 cluster.  The current running instance has been patched to SP1.  When I run setup to add the new instance I will be using the RTM software will this cause a problem?  I can not seem to find any documentation on this.  Will the new instance need to be patched to SP1 after running setup?

Any help you can provide will be great.

Thanks

Doug