When starting out I think ken combo is something people expect themselves to be able to perform and will set-up. However, when you get going you realize that the ken combo is merely the result of your opponent choosing the wrong DI that set-ups up for the best possible punish for marth (Dair) in some situations.
Therefore, I believe there to be few combos in this game that you can readily predict and practice. The biggest skill I think going into a competitive player for melee is the ability to improvise at any given situation. With those improvisations you choose one of the best options to continue your combo potential and eventually lead into a stock.
Marth's CG on Spacies on FD can very quite a bit. From choosing to tech chase them until say 20% or regrab them off Uthrow are viable options. After say 30% you might choose to regrab or turn round Utilt into regrab and eventually end with a tipper fsmash offstage at say 60% because you can get off a tipper fsmash in place. Or you might have to follow-up with Fair/Uair on far away DI from you. Despite practicing a 0%-stock on fox/falco it will usually almost always play out differently depending upon your platform set-up, di option, and possible mistakes.
The best preparation I believe you can do for combo's is merely recognizing what options you might have available to you before playing. Therefore, when you get into that situation or similar situation you can more easily deduce a good course of action. For example, why throw Fox towards the center of the stage compared to simply further to the ledge or into the air. Throwing him to the center of the stage makes your work of getting him back to the ledge all the more difficult.
Though when starting out I think its hard to do this. Thus, I say play and get feedback, then go back and reevaluate yourself. Figure out what is working, what is not working, criticize what you think might be working, etc. Find some strategy when playing XX character that you want to use.
For most match-ups of marth its getting grabs and using that as the focal point to start off your momentum. With Marth's movement in DD, Dtilt, Jab, wavedash, etc. to set-up for those situations where you might more easily grab someone. Dtilt to take space. DD (dash dance) to threaten space and at any moment go into a grab.
The game is pretty complex when you get into it.