I think that Harry gets typed as an INFP because people feel that he's a Fi user and they associate Fi more with the INFP than the ISFP archetype. Therefore, whenever they see someone who seems to be fueled by Fi, they automatically pin them down as an INFP and overlook the Se/Ne difference. Personally, I think that INFP and ISFP fantasy heroes can seem pretty similar, but ISFP heroes have usually some strong natural physical abilities and instincts (like Harry = Quidditch, Van from Vision of Escaflowne = driving mecha, swordsmanship...), whereas INFP heroes have some natural mental abilities and instincts (often it's associated with "instinctive" magic, healing etc).
In my opinion, Ron is whatever type Xander from Buffy is, and as I've read a good argument as too why Xander is an ENFP, I'd type Ron an ENFP as well. For one thing, he is definitely not a J.