It's always good to go with the safer options out of iMCF I think.
I did extensive testing on iMCF > 66A (BE) > bA and this is what I discovered:
66A (BE) after iMCF will only connect on Natsu, Tira (??), ZWEI, Raph, Damp, Aeon, NM, Voldo, Asta, and Algol. Either I'm not RCCing fast enough for other characters or it's not possible. Aeon is a unique case though. bA will not give CH properties when you use it on him after the brave edge (at least patch that shit Bamco). bA is possible with the small but very doable window on all characters you can get 66A (BE) off on. If you're getting A after 66A (BE) for bA, you're doing it while you're still in the animation for the BE. If you don't connect at all because they fell you're doing it too late. It's a tight window of opportunity but it's very doable. Even if it's character specific it's 107 (109 w/ clean) if you start with CH a:B+K. This could be why Frostheart was having trouble getting bA off on certain characters after iMCF even without 66A (BE). I guess it just has to do with how a character crumples after iMCF because it has no relation to size. Tira is a Size 2 and it works fine. I say I'm guessing it's iMCF's fault because CH 66A (BE) > bA works on everyone.