UnitID

In order to make summon spells that summon the unit you want, go to your wc3 folder... inside, there should be a units folder, right? you should have stuck your abilitydata.slk in there if you edit spells... well, inside, there is an .slk called unitdata. Inside this, in column A, is the unitID's... use them to change the unit/s you summon.. hope this is helpful!

My Unitdata destination -
C:\program files\warcraft III\Units\Unitdata.slk

for new custom units:
make a trigger like

event: after 5 seconds of gametime
action: gamemessage to everyone, text = string of unit
and you get something like

"custom_EC26"

now if you wanna use the custom unit, use "EC26" without the custom

or set a unit type varible = to the custom unit, then convert the trigger to customer text, it gives the 4 digit code right there