Welcome to the site. How long have you had the simillis?

To ensure you have a pair you should at least get 4-6 fish. If they are the same spawn then size could help, but if two spawns, then that is no help. Your odds of getting all of one sex with four fish are calculated by multiplying 1/2 x 1/2 x 1/2 x1/2. This is because each choice can be either a male or a female. 1/16 or 0.0625 or 6.25% chance of getting four males when buying four fish of an evenly sexually distributed batch of fish.

I would stick with one species per tank. Some males guard a square meter of bottom in Lake Tanganyika. Most of our tanks are much smaller than this.