imo, you take a chance any time you add 2 clowns to a tank. they could get along, or they could fight. i have gone through numerous clowns to get the different pairs that i have now. i would stick with trying a small occelaris or percula.
different types of nems in a 30g??? i would be cautious about trying that one. some nems stay put, and others will move a lot. some will stay put for awhile, and then move one day for no apparent reason. why not have multiple colors of bubble tips? that way you don't have to worry about them fighting, clowns will have a ball, and the color variety looks great, imo.
how long has your tank been established? what kind of skimmer, lights, and sump do you have on it? powerhead?