I dunno, I think MM#4 is actually important to making my point in this case. I re-read #30 just yesterday, and I found that Marco and Rachel actually clicked together pretty well throughout the book (from the playful teasing in the cafeteria to the way they kept up with each other on the final plan, despite their losing odds). The ending scene in #30, especially, showed how much alike they really are. Rachel basically knew not to BS or pity Marco about what happened, but to rather state the truth so that he could deal with the 'death' of his mother. Out of all of the Animorphs, she was the one that connected with him the most at his weakest point, and I think that's pretty important to consider because of how well they understood each other. From a strategic standpoint, I think they both saw things a bit similarly during the war also, so they both really had a lot going for them had they been paired up during the book series.
Anyways, the reason I'm saying all of this is because I feel the match up in MM#4 with Marco and Rachel wasn't completely outside the realm of possibility of Rachel being interested in Marco. Tobias, as you said above, always felt as a pity case to me when Rachel showed her feeling towards him. It's a relationship that I didn't really root for because I never really understood why Rachel would be attracted to Tobias other than pity. I mean, sure, she probably thought he was brave for fighting while living his life as a hawk, but even that is mixed with pity also. They really didn't have anything in common for the relationship to work, whether he was hawk or not.