as a rule - one i'm sure that you know, but for everyone else - bare feet, skintight clothes.
there are a few descriptions of the anis morphing outfits, but it occurs to me (now that i've taken a moment to think about it) that the animorphs, being 13-ish to 20-ish over the course of the series, would have ample opportunities to switch out the outfits as they got older. would rachel have stuck with buying new leotards after quitting gymnastics? (not that this detail is mentioned, but i can't imagine that she kept it up through the series.) in either case, the girls are typically described as wearing leotards / one-pieces. the guys are typically described as wearing bike shorts and t-shirts.
suffice it to say that i think that you've got plenty of room to take artistic license. go nuts.