Hy B.,
that's great you're planning his first birthday now and taking such concideration in other people's schedules.
you might want to host a "toys for tots" party in honor of your son's first birthday. people who are giving during this time of year are a little more inclined to do for those less fortunate. have one box guests put the donations into and more likely than not, there will be a few guests (like grandparents) who bring the birthday boy something special, too! we did this for my son and it was SO much fun!
as far as guests all getting along, i hope they're all adults. your concerns might be well recieved if said right. suggestion: "hey, i know it's akward for you to be in the same room with so & so and his/her date, but it means a lot to your grandson that you're both here today!"
sometimes honesty does work wonders if it's executed in a tactful manner.
good luck and happy b-day to the little guy in a few months!
D.