Limited URs (promo URs obtained from the limited scouting boxes in JP like #1319, #1417, and #1425) are able to have more than 2 SIS slots (I believe they start with 4?) but it's not letting me input more than 2 skill slots for these cards.
I think it'd be better to have a separate category for these cards rather than "Promo" because they're more powerful and are scouted rather than given out.