You can uses a chamois cream for longer rides to avoid chafing but you don't really need anything for shorter rides like CX. The synthetic chamois/pad absorbs sweat and kills bacteria better than anything you could wear between it and your sensitive parts.
no underwear with cycling shorts, dude. No vasoline, either. If you get saddle sores from training, then start using chamois creme. You can avoid saddle sores by having the shorts fitting snuggly against your crotch. I've seen many Cat 5s walking around with their chamois hanging low like they think they're wearing jeans or something.
Further - if you're getting saddle sores from training, your saddle might be an issue as well. Chamois creme really helps when you're spending a day riding. I know it's $$ but if you can buy a few brands shorts to get a feel for the chamois you like, it is totally worthwhile. Also, sadly, the more you spend (in a lot of cases) the happier you'll be.
personally I've never needed anything more that just the pad in the shorts. I'll echo the others that you should just make sure they fit snugly. You'll find, if you haven't already, that they're actually quite comfortable for riding a bike. Almost like they were designed for it...
I should also mention that I resisted wearing the bike shorts for a long time and glad I made the switch. It took me a long time to convince my wife not to wear underwear under her shorts. All it took was a 40k time trial and she was sore enough that she decided maybe the underwear wasn't such a good idea.