just give him a big piece of banana rather than chip shaped in future, although you may have to leave the skin on the ends so that he can hold it. i don't think you did anything wrong at all, like colditz said he's learning how to gag and control food in his mouth so it's just a learning curve they go through.
as for the mucousy vomit, i have rather geekily written about this on the blog here and i personally think it's a clever thing their body does to lubricate food that gets a wee bit too stuck. amazing, huh? (of course it's speculation on my part, but the only time the mucus came up was when she did a bad gag. like, twice in total or something.)
as for the worrying... nothing you can do about that. you know he managed fine first time round, his body did everything it was supposed to do, but i think it's your job as a mother to worry so your stuck with it...