You can't do that all in a single Smartlist, unless you have a way to filter the Podcasts that you do want limited by some characteristic they all share (like [Genre] or something). But it would be easy to do with two smartlists.
Smartlist 1 (the Limiter):
[Media Sub Type]=[Podcast] [Album]=[Podcast Name #1],[Podcast Name #2],[Podcast Name #3] ~sort=[Date]-d ~limit=-1,1,[Album]
Replace Podcast name #1-3 with your own podcasts that you want limited.
This list will result in just those that you want limited, and only include the single most recently released episode of each.
Smartlist 2 (the Shuffle):
Now, for smartlist number two, I can't give you exact code to paste in, because it depends on what you name #1, but you're going to do essentially this:
1. [Media Sub Type]=[Podcast]
2. [Album] is not any [Podcast Name #1], [Podcast Name #2], [Podcast Name #3] (the same list as above, but we want to exclude them).
3. [Playlist] is any the Playlist you created above.
4. Add a shuffle modifier.
Now, go to Import/Export on this playlist, and surround items #2 and #3 with parenthesis, and put the word OR in between them.
Now, this list will result in a shuffle of the contents of Smartlist #1 AND the rest of your Podcast episodes (so those you don't want limited). This is the one you actually play or sync or use or whatever.