Well yeah there is a lot of information on core training but the core training GUStrengt has is performance based. The basic philosophy is to eat right (I am the only one on the site who doesn't) and train for performance. The guy who own GUStrength, my mentor: Gulf War vet Eric Troy is like 195lbs @ 7% bf with a 550 Deadlift...so, like I said: it all boils down to your goals.
You lift to look good so I think your primary preference should be to stop thinking like a strength trainee. You are a bodybuilder. So you need to plan a layout like this:
1.) Do a strength exercise for 1-5 reps and progress using the Single, Double and Triple Progression I mentioned earlier.
2.) Do 1-2 other accessory lifts for 8-12 reps because they will be more aesthetically beneficial. For example: incline dumbbell press and shoulder press, etc.
3.) Do 1-2 isolation exercises for 2-4 sets for high reps and superset them. Like: incline flyes and dumbbell curls.
Your template should be:
Day 1: Lower Body 1 - go hard on this day. Heavy deads, rows, etc..
Day 2: Off
Day 3: Upper 1
Day 4: Lower 2
Day 5: Off
Day 6: Upper 1: lots of isolation on this one
Day 7: Off
Day 8: Off
Day 9: repeat
As you progress (and by progress here I mean solely looks because that is your basis) you will notice subtleties individual to YOU which you can address.
Regarding flexibility: there are BUNCH of articles:
Flexibility and Mobility