Are square moon signs enough to ruin otherwise good compatibilty?

No a square moon is not enough to upset compatibility. Having your house cusps in the same sign, I find that creates a plus or a minus. My hubby and I have the same house cusps off by 1 degree. After being together for 36 years, I can assure you that this will bring about blessings and hardships. When things are good they are very good and when bad, they are a challenge. All unions need a little challenge in order for you to grow. Without that square, you could easily get bored with each other, or simply lose interest. Therefore you both will go through the transits by houses the same. By becoming aware of this, you will either work together during those tough transits, and progressions, or not. But it is definitely creates am experienced of being together.