This is my absolute best matchup, huzzah.
First, don't dash attack when either of them is on the ground. It can be used to intercept them coming from the air, depending on the IC's preferences for their air game, but not well against on the ground. Not only are IC players *extremely* shield-grab happy, but there's usually a better option anyways. Wavedash-ftilt is much, much safer in this matchup. Keep it spaced right and if they shield it there's no way they can get a grab.
If there's two of them, don't grab. There are some Samus' who do grab in this match up to some degree of success, but they're silly. If you do, immediately dthrow in the hopes that you can hit the one you didn't grab with the one you did. But, honestly, just don't. If it's just Popo, however, considering how much they're usually glued to the ground grabbing can be quite feasible.
You're spending a fair bit of time in the air. IC's can move on the ground far, far faster then you can in the air - they'll usually be able to run under you and jump uair or bair, or just hit you as your landing with a smash, or grab... they have lots of options. It's not a good place to be. If you do go in the air, get back down asap. Don't let them get under you.
When recovering, if the IC is throwing ice blocks predictably, you can actually airdodge through them to grapple. Some IC's are really trixie, though, and will make this very hard. You opponent didn't seem to do this, so you could have airdodged through. You can also fair (and uair?) through them quite well if you get the hang of it. Or, as you approach the iceblock stream try to time a bombjump to go through them. IC iceblock spam can be annoying, but Samus should be able to get through it fairly well if that's all they do.
You opponent didn't do this much, but many IC's will desync during the match with a sidestep or roll - this is an excellent time to punish them with a charge shot if you've got one.
If you're sufficiently far away, spamming is fine. As they come in close, either chase your last missile and switch to offense or keep off of anything laggy. Don't keep spamming as they're in close, and don't try to jump over them to go to the other side. These are both times they'll punish you.
As Mars said, once you hit them, follow it up - don't run back away. Gotta be offensive when the opportunity presents itself.
Your dtilt and dsmash are your best moves to separate them with. I saw you use this some... use it more. This works great to catch their wavedash approach if you call it or time it properly. Follow with a bair or uair if you can, this will likely knock one of them away. Once this happens, keep following one of them up (keeping in mind Popo will try to stop you, so don't do anything to laggy to Nana if Popo's close enough).
If you can get them to both be on opposite sides of you, fire a (missile-canceled) missile at Nana, who will be dumb enough that you should be able to do with a reasonably high success. Then deal with Popo. If the opportunity presents itself, fire at Nana again. Just keep her back with projectiles and stay between her and Popo. Usually the Popo will be either really aggressive about getting back to Nana, which you can punish, or lax, in which case you can usually finish Nana off.
Not that you seemed to be having trouble with this, but figured no harm in mentioning: edgeguarding - keep in mind Popo can't call Nana to up-B if they're too far apart, as well as if Nana's just been hit. When they're far enough out, throw stuff at Nana - she'll be too dumb to dodge it without Popo's intervention, which will leave them *both* vulnerable. While the opportunities may not be plentiful, spiking IC's is far, far more effective than it is in most match ups, because of how horrible Popo's virtual recover is without Nana.
I'm sure there's more I can't think of at the moment, getting mighty tired, but I hope this helps. In summary:
(1) Don't dash attack if one is on the ground, if at all.
(2) Minimize time in the air, don't let them get under you.
(3) Don't be so completely defensive - if they get to close, don't run away or keep spamming, but chase a missile or stick with a ground game.
(4) Keep them seperated.