During marathon buildup...good to race everything from 800-1 mile to 5 km 10 km too?

No. Just run 800s and miles if you want to for fun. You don't need to justify it by telling yourself it's a key part of a "marathon buildup" - it isn't.

An 800 m or mile race won't help your marathon performance. It won't harm, either, if you don't skip too much marathon specific training for this. However, if you do too much specific middle distance training, cut back on mileage and marathon specific workouts and peak for these short races, then it might be counterproductive to some extend...