If you really will need 5 mics sometimes then you'll need a mixer with 5 mic inputs. This may change your options for mixers--quite a few manufacturers make a jump from 4 up to 8 mics without much in the middle. Frankly, an 8 mic input mixer might be a good idea. If there's one thing I've used about mixers over the years it's that you always need one more input than the maximum you thing you want!. If you do decide you need an extra mic, another mixer you could try would be the newish Soundcraft SIGNATURE 10. It's a bit more expensive than your previous short list but it's a lovely mixer. Do note the restriction that this will only give you a single stereo out, not multiple channels.
As for bleeding between mics, three things to consider. First, make sure your mics are cardioid or hyper cardioid in their pattern. This means that this pick up most from straight in front, with the pick of diminishing rapidly as you move around the side, down to almost nothing from behind. Second, your best arrangement from a sound point of view will be a largish round or multi-sided table to make the best use of the mic pick up pattern. Here's a picture of a BBC radio studio that sort of illustrates what I mean--the beeb is probably the best there is at doing on air discussion type shows.
Finally, for radio, you want your studio to be as dead as possible acoustically. This is a bit different from a typical music studio where you want the room to sound nice...for radio you don't want any room reflections at all. Pro acoustic treatment is expensive but basically surround yourself with as much soft stuff as possible. Also, on mics, if you buy dynamic ones instead of condenser, they are less sensitive and pick up less background noise (but you need to speak a bit louder).
That's what I do. I drink and I know things.
-Tyrion Lannister (and Bobbsy)