Rather then only going into training mode and practicing new techs and new combos, go into the replay section and watch old replays you have saved up of you playing with your two mains you listed above.
Here is the mindset you should have in this case:
"I'm watching a replay where I lost to Sheik as Little Mac. How could I have lost? What did I do wrong? Well from what I have been seeing in the replay, Sheik seems to have a lethal advantage once she 'short-hop Fairs' me which can lead into some long combos that are difficult to break out of and rack up devistating amounts of damage. So how can I avoid this? Well I know Mac has incredible ground speed so maybe it would be best to take advantage of that and be on the defensive along with using my shield. I should not risk gambling a potential stock by playing too much on the offensive when it will be punished by a move like Sheiks Fair or Ftilt which can come out VERY quickly."
This is the mindset that you should have. Remeber, replays are your friends. About 75% of the time, what you learn from a replay is something you can't take to training mode. In my opinion, getting familiar with matchup stradegies is the #1 thing you should do before going into training mode. It's one thing to learn TECHs and combos but if you don't know what situations to apply them in, what good are they?
For example, I can go into training mode now and learn perfect-pivoting but after learning it how am I suppose to know when to use the TECH if I havent discovered a need for it?
Right now, I am learning the BIDOU TECH mechanics because based on mistakes I have analized on replay's while playing as Little Mac, I have discovered that spacing is a giant priority in playing effectively as Little Mac especially against high-tier characters like Diddy Kong, Cloud, Rosalina, Sheik, etc.
Hope this helps man! Good luck to you!