As a male...
I don't think it's suitable to grow nails. LoL. Anyway, since I'm also a feet man (I have both a foot fetish and a tickling fetish => they're propably related to a persons early tickling experiences), I'd say the tongue also is a fine tool, whenever a girl's feet become wet they also become ticklisher, but that's also dangerous to use (because she'd also be out of control).
What also helps is, when you wash her feet, when they are soaped and then fully cleaned with water, I'm telling you, just out of the shower/bad is pure tickling fun assured, and it makes her smell good too.