There are some tricks for scouting further, like taking out the Tachyon Guardians on each planet, or sending your scouts in transport ships (Be warned, transports apparently alert literally everything on the planet). I'm sure other people can give you a bit more info on that, but all I ever really do is snipe off Tachyons.
Re: The AI's strength, you should defend your planet the same way you usually do, since generally speaking the waves will be distributed across all the players and the AI won't necessarily send a dominating force to your home planet. Granted, it can, but if you didn't get a proper defense up then you can force your friends to help you instead! Teamwork!