First of all, I definitely support starting off with a much smaller mission as your first project. It's a good measure of how much time and creative energy a level pack requires without as much chance of putting too much stress on yourself.
Just played through the level on Ace and didn't have a whole lot of trouble, except that the Smelter shooting missiles was a nasty surprise. There wasn't much space at all to dodge in that room, so try to keep factors like dodging space in mind when placing robots.
The other thing to mention about the robots is just to be mindful of certain weapons when you do the modding. If you give a robot a player weapon, it will generally do the same damage on every difficulty, where the built-in robot variations are a lot weaker on lower levels. The Level 1 Laser does 9 damage per shot, which is a little high for a "weak" robot weapon on the lower skill levels.
Otherwise, the level looked pretty good aside from the exit door being on a flat face. The level was probably a little small to have two keys, though, so perhaps you could change the area where the red key is now into a side area with stronger powerups, and have the red key take the place of the blue one, but that's only a suggestion, of course. The texture scheme choices were unconventional, but in an interesting way, and the architecture and detail was very
good for a beginning designer, so I'm strongly encouraging you to keep it up.