Hello!
I started the process of building my first frame and I'm currently working on the bike's geometry. Can you please tell me if it is possible to use different tubing from different manufacturers on the same frame? (Say Columbus and Cinelli) Or at least using different models from the same manufacturer like Columbus Spirit and Aelle?
I am asking this because I want to build a track bike and the tubing I find is not consistent. I want to use Columbus Spirit yet I cannot find Spirit forkends. I also intend to use 1.1/8" steerer headtube.
Thanks in advance!

If it is steel, you can mix it together, regardless of who made it. Most tube manufacturers don't make dropouts or other frame parts, so pick any dropouts(forkends) that will fit the tubes you plan to use.

Many years ago it was a good sign that a brand used a complete tube set, like a complete 531 Raleigh International. Then came (at least here in NA) custom builders who mixed and matched tubes that resulted in a frame better matched to a specific rider and their needs. (I think it was Bruce Gordon among those who first got attention for a lack of tube decals on completed frames). Now using a complete tube set on a frame suggests a certain economy to some.

Mix and match steel tubes as needed. As long as they compliment the goals there's no downside (other then that decaling thing) BTW Cinelli never made tubes but has made pretty nice fitments (excluding their lugs which take quite a bit of work to look nice, IMO) Andy

most currently available tubing decals look too gaudy to me. It all started going downhill when Reynolds put that box of fine print on their decals in the late '70s. Tubing decals used to be important, there was a recent thread in C&V about the "CroMo Seat tube" decals, meaning all the other tubes were gaspipe.

Can I use my own dropouts? I would like to create them by myself since I have a wacky idea, but my only available material would be just plain steel. Would it be okay?

Paragon uses both 4130 and 1018 steel for their dropouts.