Well, I currently play mono and I've only played a few matches, but I noticed that my deck (or maybe Piranas in general) don't have many answers to SOA. I guess they're also hurt by a lack of a good DR.
Would a half deck be more effective? If so, what would be optimal to pair up with Piranas? I was thinking Vortex might work nicely with Piranas.
Why? Piranas' best characters characters are strong early; same with Vortex. No, it's important to pair Piranas with a clan that is strong late, such as one with a combat bonus or SOA. You also want DR and SOA from your other 1/2; you want to win with your Piranas characters after all, so you want some characters that you can lose with for your other 1/2.
Would Piranas/Junkz be a good combination? I found this preset that seems solid: http://www.urban-rivals.com/presets/?id_preset=2511589&list