Yep, totally okay to do. All Intel-based macs support dual-channel memory, so in a nutshell: You'll get the best performance with equal-size RAM sticks, but it won't kill it if you don't. Luckily, most RAM is sold in pairs these days.

Is it ok to mix 2 GB and 4 GB DIMMS in an iMac (mid 2011)?
My question is how to create multiple raster layers group, for example when using several aerial photos. I wish to group the layers into one so that I can edit/select as a batch rather than having to click on individual layers.

I use ArcGIS 10.2.

Thank you in advance!
