I have now seen a couple of good chaingun moments. Such as "I know that this wall will slide away to unleash a horde of demons, shotgun zombies and imps." They start in a cramped space, I can fire from the flank, essentially requiring them to pass through a wall of hitscans to get a line of fire, and when the worst is over I can step forward and fire in to clear the rest of them before I'm out. Then the duplicate horde comes charging over the symmetry-line, though, and my gun just ran out...
The things that get stuck in ceilings: Other people have reported such things recently. It may be caused by a new zdoom version rather than a new HD-version, since such things have been reported with different mods. (Gibs hanging about, puffs in the ceiling, shards in trees...)
So when did the stuff getting stuck start happening, and can you make it happen without HD? Maybe check/post in the bugs-forum.
NPC Allies: Another option would be to summon them 1 at a time. Maybe even (for style) have 3 different items to show in HUD. (Starting with the 3-symbols, replaced with 2-symbols when used, then 1 symbol, each use beaming in 1 marine.)