We meet again Scotu.
It seems we have the same problem with comboing with Falcon. =(
I will try to give combo advice. . . . .
1st vid. You had the right idea of upthrow to start combos on Peach but only do that at low percents like up to 30 at most. You should go from nair chain (things) to upairs to knee at low percents and upairs to knees at high. Your upairs wouldn't go down to the ground fast enough to chain them which is mostly a problem with timing the upair and fast fall. The stomp lcancel on sheild is all timing since the stun from hitting the sheild is signifigant. Just remember to do l canceling later than with most arials (you could also try to fastfall at different time depending on preference to make it easier).
2nd vid. With pikachu, just upthrow until I guess 50ish maybe 40. When you do downthrow, try to get him near the edge and knee. Mostly, you just need to get him close to the edge so you can knee him at around 70% (remember near the edge) on FD or so. It is better just to go with one upair to the knee close to the ledge since he is so light. (My opinion on this combo matchup thing)
3rd vid. Always downthrow unless at really low percents. So when you see popo alone you 1. grab 2. down throw. 3. upair 4. knee. You can get downthrow to knees way up in those percents (I think till 100% but not sure). They are light, so you should knee a little bit more to seperate them and edgeguard if you can (much easier than other characters). On a sidenote, overb or foward b is more useful in this matchup due to nana sheilding a little bit after popo sheilds. You can usually hit nana (since she is stupid) with the foward b and seperate the ICs with the knee.
4th vid. The whole matchup is techchasing. I don't know what else to say besides look at opponent patterns and blah blah blah. Although, you should consider using downsmash as a techchaser. It is actually pretty good especially near edges. I am guessing that you didn't take knee oppurtunities because you were messing around. I will say it anyway. . . . . . . . .just take do the knee like you would do on light characters.
fox vid. Same matchup as Falcon. . . . . . .except fox has more priority and nair and shine eat through everything. You did well in this match so eh. I don't know. If the fox is at 70%, I would probably give him the knee no matter where he is.
falco vid. DARKRAIN COMBO!!!!!!!!!!!!!!!!!!!!!!!!!! ish thing there. You could have kneed him at like 60% and he would have been dead much sooner but I guess you wanted to be sexy and you suceeded. Remember that Falco is light. A knee will probably kill him 50% to 60%.
General stuff: Know your throw to knee percentages. On most opponents, it is better to combo them to the edge then knee them when you get there (Samus and the like). You need to time the fastfall with the upair chains and how to space them (if you know what I mean by the spacing upairs thing XD).