HOW DO I MAKE CUSTOM UNITS SO THAT WHEN I BRING UNITS TO A LOCATION, THEY WILL DIE AND SPAWN A NEW CUSTOM UNIT?

HOW DO I MAKE CUSTOM UNITS SO THAT I WILL NOT EFFECT REGULAR UNIT BUILDING?
*****EG I CAN STILL MAKE REG MARINES AT BARRACKS, BUT WHEN I BRING THEM TO AN UPGRADE POINT THEY WILL DIE AND SPAWN A CUSTOM UNIT THAT I'VE PREVIOUSLY MADE STATS FOR.

HOW DO I EXCEED THE 200 LIMIT PER PLAYER?

WHY CAN I PUT IN FLOOR GUN TRAPS ON MY MAP?

Way to go all caps.

You might want to just go and mess around with the editor before you start asking questions, you'll learn a lot better and you won't waste anybody's time.

In order to do really neat stuff, you'll need SCMdraft, but since you seem brand new, just use the regular editor until you get a basic idea.

Lets start with this. "How to exceed Supply limit"
Simple, spawn units with triggers rather than building them at a building

"Gun Traps"
Get red of Staredit, and use SCXtra or SCMDraft (Or starforge, but hey, nobody knows about the corruption between it and Emerald Placer)

Don't write everything in CAPS... its annoying.

C: Current Player brings at least 1 <unit name> to spawn
A: Kill 1 <unit name> for Current Player at spawn
A: Create 1 <other unit name> for Current Player at spawn
A: preserve


Same answer as above. Just specify how many units of which kind will be replaced by how many units of which other kind.
Oh and for changing stats go scenario -> units in SCMDraft 2

Tutorial Link: Modifying Supply Limit

I don't know why you can't - I can...
Maybe elaborate?

Damn, I learned the basics by reading the StarEdit help. I was like 12 and didn't understand much english, but I managed to get what a condition is and how it works with the actions.

Use hero units to replace the units, they look the same but are different units.

However dont sound the same, give the same kill score, and start with all reaserch.

Some do sound the same.