You should be able to go to Simple:Press admin > Manage User Groups and then under the Default Usergroup for moderators of the forum click on add members and select a member of another group and add them to it.
That is assuming you have a moderator group already in place. If not then you would need to create one. Is this what you are trying to do or are you getting the error elsewhere?
Regarding the font size there is a default size set in your SP theme that you can change. If you are using one of the supplied themes with an overlay file you could change the $mainFontSize entry, in your overlay file, from 100% to a larger value (try 105%) and then go up/down from there.
If you are using the CSS-Only theme then you would change it in the css-only.css file.
If you are running one of the supplied themes keep in mind that any updates that come out will overwrite any changes you make so it is a good idea to create your own custom theme.