Cats and guinea pigs usually get along fine. Most cats either completely ignore them or are scared of them. Some like to watch them. Several of my cats would lay on the cages and watch them. Carmel liked to sit in the hayloft with the pigs when I let her in the room. 1 reason cats don't see guinea pigs as prey is because guinea pigs are fairly large and they usually aren't that scared of cats. All but 1 of my guinea pigs has always gone right up to the cats to see them and if the cat doesn't move start cleaning or trimming their fur. The only 1 that ran away is also the only one that has gotten chased by Carmel's bold kitten.
For a latch try ordering snaps used on horse tack. Few animals accomplish opening those and they kept Carmel from eating my gerbils. She can open most standard small animal cages and some doors. A big double sided snap also worked well to keep my houdini horse from opening the gate until he learned that running into with enough force would break the snap. A cat shouldn't be strong enough to break even the smaller snaps.
These are simple for humans to open and hard for many animals
http://www.valleyvet.com/ct_detail.h...2-00b0d0204ae5