but when I did it in multiplayer the others were all dropped by decyz? or something

the condition was When I click marine and action was create hero marine

but whenever I do this I'm always dropped

is there any way to solve this problem?

No

Someone else pointed out that unit selections must be networked (synchronized) or else team control wouldn't work. Is that true? Has anyone gotten EUDs for unit selection to work without desynching?

I don't think that's true. The unit selection system is based on control groups, which are not shared. This fact can be seen when watching replays. Depending on the player slot you have while watching you can or can not see a player's hotkey assignments.

I suppose when playing team melee not the control groups are shared, but the unit orders.