I think it's an event like gold fish or you need to donate to get that pet...who knows... If you don't have runes right now you need to buy them...
I have all Rune slots filled with runes at least level 3 or higher. I don't think I have any level 5+ runes, however. . . Maybe ONE. . but that's tops. . . .
I'd personally go with runes first until you feel like your summon is a weak link. I guess that's not very helpful? But since a purple summon with full runes takes you way higher than an orange summon and also costs less...