If your mysite site collections are in the same web application as your other site collections, they will share content databases. Unfortunately there is no way in specifying which site collections go into which content databases.
This is one of the reasons people like to put mysites in a separate webapp. Nowadays, with the rise of "host named site collections" and them being actually usable now, that idea has perhaps lost some followers but it's still a valid approach.
If multiple content databases are assigned to a web app, a round robin mechanism will be used when creating sites. The only thing you can control here is specifying some content databases not to be used anymore (by setting the maximum number of sites), but that will be true no matter what site collection you create.
The "self service site creation" property is scoped at the webapp level, so you also can't do this at the individual content databases.