I'll take a look at the linked characters and will let you know what I can find, thanks!
Regarding characters that work, these are the characters I tested when I made the initial Vita port: https://pastebin.com/e4eyG4Yb
There are a couple of anime and touhou characters in there, some of my favorites would be Dio from the Capcom fighter (https://mugen.fandom.com/wiki/Dio_Brando ), RP's Mima (https://mugen.fandom.com/wiki/Mima/RicePigeon%27s_second_version , link in the pastebin died), and some Otokojuku characters like Edajima Heihachi (https://mugenarchive.com/forums/downloads.php?do=file&id=106661-edajima-heihachi... ).
Adjusting the amount of characters should work by adjusting the characters available in the select.def inside the data folder. The default select.def that ships with the basic Mugen download (for example available here: https://mugenarchive.com/forums/downloads.php?do=file&id=5283--official-mugen-1-... ) has an explanation on how adding characters to the file works.
Additionally, you might have to adjust the character select screen. This is done in the system.def file used, by default that will be in data/system.def. Under "[Select Info]" are a variety of options, for the character amount the most important ones are "rows" and "columns", which define the amount of characters shown at the select screen. So if some characters aren't showing up, you might have to increase these values. I think that should be enough to include any amount of characters, at the very least more than eight. Unless this was a question if there is anything special you would need to do for Dolmexica to show more than eight characters, there is nothing special there, that should work just like Mugen.
Thanks a lot again for replying again and for the links to the character links, I'll take a look!