"Expanding too much" is still based on systems (not planets) within a number of turns. Off the top of my head I think it's about 3 systems within 10 turns, adjusted by difficulty setting.
I think this is the previous (old) thread about it:viewtopic.php?f=7&t=1143&p=5032&hilit=expanding#p5032
For your second point, the AI will not usually colonize a planet in a system where another race has colonies, the exception is when the order to colonize is already in progress and another race beats them to the system.
If neither of these are meeting your expectations, feel free to send me the reports and I'll take a look at them.