Flite Test recommends an "1800 - 5000 mAH 3S" battery for the Guinea Pig.
On my bigger planes (a FT Sea Duck and an
Ansley Peace Drone), I like to run dual 2200mAH 3S batteries. A parallel connector is pretty easy to make for yourself. It's only a couple bucks for
a pre-made connector if you don't feel like messing with it or don't have spare XT60 connectors laying around. When the packs are connected together, they just act as a single 4400mAH 3S battery for both flying and charging purposes.
This gives me the option to use the packs separately in other mid-size aircraft, or together in the bigger aircraft. You could also opt to just run a single battery if you wanted the extra payload capacity for dropping candy or something out of the Guinea Pig. Just keep your center of gravity (CG) in mind if/when you fly with a different number of batteries. Either the battery tray needs to be right on the CG, or you need to adjust the pack(s) forward/backward depending on how many you're using.
Not to make this too complicated, but I'll also mention that if you choose to run a single pack at the lower end of the suggested range, you'd need to keep the C rating in mind. The low cost batteries I typically run are rated for 25C. Depending on the brand, that rating can be a bit "optimistic", too. An 1800mAH 25C pack is only rated for about 45 amps. That pack may not be real happy about flying a decent sized twin-engine plane like a Guinea Pig or Sea Duck. Once you get up into the bigger sizes (or parallel connected smaller packs), you can get away with running the cheap 25C packs.
