I don't have a Grendel but I think you can stuff a few rounds in a regular 5.56 mag and it will feed. Any more than just a few you are going to want to get a 6.5 Grendel magazine. Otherwise yes, you can use a 5.56 lower and mate it to a 6.5 Grendel upper.
I've heard Alexander Arms and Odin Works make pretty good 6.5 uppers.
Texas A&M Class of 2011