Well, I've never worked with it, but from what I know of e-smith and what I've seen on the Plesk page, I think it'd be a bad idea. e-smith 4.1.2 is based on RedHat 7.0, so Plesk would probably install OK, and quite possibly even run OK. However, you'd likely see pretty much the same problem as arises with Webmin--Plesk won't know anything about the e-smith configuration database or template files, so any changes it makes are likely to be overwritten by e-smith (or worse, leave the system in a partially-unusable state).