Hello, Dale! Some wonderful questions (before I found WetCanvas, I had the figure it ALL out myself!). First, I paint with watercolor....I find it easier to draw my painting and transfer it to watercolor paper. I use a larger sheet, then cut it down..that way I can extend the background and can use a mat to decide on the best way to cut it down. Plus, I TRIED real hard to paint on a cut-down card...some folks do...NOT ME..to me, I may be feeble fingered clumsy..don't know, but it is much easier for me to manipulate a little bit larger piece of paper (plus, my painting with watercolor makes it less likely to buckle if it is on a larger piece of paper!)
Mats...there are places on eBay that will sell you precut mats that an ACEO will fit in to. If you PM me, I will give you the site. With a mat already precut for ACEO size, there is no problem with trimming down to ACEO size. If you like a white border, you may incorporate it into the ACEO itself..that way, if you choose not to mat there will still be a "mat" around the picture. I'm thinking, not a bad idea, cause one can sure run out of ways to paint a background...that is the toughest part for me, trying to figure out what the background will be...and as you draw smaller, oh my, you WILL find you need to fill all that SPACE!!!
Always square corners...some people prefer rounded, but not so good!
With ACEOs one can use any support...from Masonite to Paper to Wood for the, what do you call it, burning your art into the wood...so the world's your oyster! I use 300 lb. to 200 lb. Hot Press, Sometimes Cold Press...there are some sites I've read that say only to use smooth paper. There are other sites I've read that describe "miniature art" as ANY art done in a miniature scale even if it is part of a larger painting (!!!???***). Have NO idea of what thas is about, since it seems to nullify the idea of miniature art!
As far as medium, anything goes there to! Pastels, Watercolor, Oils, Acrylic, Gouache, burning tools, carving tools...the list goes on and on!
So glad you have plastic sleeves! That way, you can sell or trade an ACEO that is protected from hands, atmosphere, etc. Oh, a note, it is best to finish your ACEO as you would a larger painting (Sealer for Acrylic, Varnish for Oils...me, for watercolor, I seal with an Acrylic spray that protects from UV rays since many people do not put ACEOs under glass...plus it protects them from handling, humidity, and ACCIDENTS!
Anything else you want to know, I can help answer, or someone else in the forum will be kind enough, I am sure!
Welcome to the LARGE World of SMALL Art!