I really like Michael, and I love how Michael "Mac" Allan is meaningful to your families.
I don't find Mac to be all that trendy. Okay, maybe for girls named Mackenzie, McKenna, ad infinitum, but many of them go by Kenzie, Kenna, and so forth. Mac on a boy has a totally different vibe to me. It's sort of a homey, earthy, trustworthy feel. I may not be making much sense. :(
Even if I'm wrong, and Mac is the trendiest of the trendoids, it's not so bad to have a trendy nickname as long as you have a solid, classic proper name to back it up. Michael Allen certainly qualifies.