I'm no expert, but will share my experience with painting enamels:
I have to admit, I haven't met a painting enamel I didn't like. Although each form has it's own quirks. I started experimenting with them a couple years ago and am still playing. I have all the types offered by Thompson: watercolors, acrylics, ceramic pigments, overglazes, etc.. And I have all their painting media. I try to first pair the media with the painting enamel based on the description in their catalog then experiment from there. Water based media is my first choice because it dries much quicker than oil based ones. Acrylic paints are easy because they are vivid and can be used right out of the tube and if the tube dries out I just cut it open and reconstitute with distilled water.
I also use china paints. They're made for china pieces-obviously-but since they work on glazed surfaces, they work on enameled pieces. I'm guessing these are similar to overglazes. They come in tons of colors which is why I love them. If you look on the different china paint websites, each offers a variety of painting media both water and oil based. One of my favorites is pen oil. This seems to dry quicker than other oil based media and some are called 'paint over' which lets the oil dry and then you can paint other colors over it. I like to mix china paint with pen oil, apply with a quill pen (like sketching) and either fire and then fill in the blanks with color or sketch with the pen oil, let dry and fill in with other colors and then fire just once.
I use overglazes from Vitrum Signum in England. Once again because of the many colors available and mix these with water based media.
I recently purchased quite a few mason stains from a ceramic supply but haven't yet tried them. I'm guessing that if these are for ceramics and glazed surfaces, they can be used like the ceramic pigments that Thompson sells. Not sure if they need to be mixed with a painting flux or can be used straight. I've also used some watercolor pans from Amaco, a ceramic supply company. They are ceramic overglazes in pan form that can be used over enameled metal surfaces and are mixed simply with water.
I fire at 1400 and keep an eye on the progress-I have a window in my kiln door. As soon as it flows I take it out. Reds, oranges, pinks and others in that color range are touchy and if I leave them in too long I lose them. If I'm firing multiple colors I try to fire the reds/pinks/oranges separately/last to have control and not lose them. Some colors do sink in and some don't like to be covered by a final clear top coat. I should really take notes for future reference. In all cases I apply the paints over a fired enamel base.
Hope this helps. I've never used Shippo or Vitrearc. Hmmm...looks like I should be looking into these.
You will have fun. Etching is one of my favorite things to do. I use ferric chloride but I know there are other methods as well.
As far as sinking, I've had that happen with my black china paint mixed with pen oil. But it's minimally visible. When I fill in with colors I usually grab any type of painting enamel that appeals to me and mix with one of a number of different water based media. So maybe that's why there's a difference in the surface characteristics. A final clear coat usually gives the piece a uniform finish.
With some of the lighter colors and whites I sometimes have to give a second application and firing or they seem to fade out.
With the clear final I use a very soft firing enamel, fire at 1400 and for a short time--only until it glosses. If fired too long and/or hot I can lose some colors such as reds and pinks.
The piece I had the problem with was done with a layer of hard fusing white with a design on top of black china paint with pen oil for lines, filled in with the Amaco soft pan overglaze. Then I put a clear final. I got separation and 'floating' of the painted layer. But if I used a hard fusing top coat and the paints are softer, that would explain it. Operator error. Unfortunately I never make notes but I know it's a good thing to do. I'm usually having too much fun to do record-keeping!
By the way a final coat isn't necessary--I use it if the painted surface is uneven and I want a smooth surface.