you can only alternate between 2 skins using the skins manager not several..there's also a separate macro extension that enables this feature if you don't want to use the skins manager.
it's recommended that alternating skins be similar or related due to the nature of kmeleon skins which don't have strict rules on what toolbars should or should not be visible/existing.. enabling multiple skins changing will cause ui problems in missing buttons or big gaps between toolbars when a skin includes certain toolbars that another one lacks.
i don't think there's such an extension that automatically changes the skin in firefox or opera..you got that confused with personas, those only change the skin background and not the actual skin and no such extension/widget exist for opera at all.
if that's wot you mean, there is an extension called kmoods which is similar to personas:
http://kmeleonbrowser.org/forum/read.php?9,104168