From what I have read the s900 will draw 100amps max and 55amps typical.
The AS150 will take 150amp while the EC5 will take 120 amps. The best option would be to change to all As150 connectors as it will give a bit more margin but both should be ok.
If you want to use the EC5 then changing the As150 to EC5 would be better than using an adapter as there is one less point of failure.