You can easily get 50 amps into those batteries when low on charge. If you expect to actually get 50 amps your panels would have to actually produce 700 watts. Reality is with sun angles, clouds and temperature etc your 850 watt rated panels may only produce 50% to 90% depending on conditions.
Need more info on the 250 & 300 panels to decide if it is better to have separate controllers. Basically if the cell counts (60 or 72) are the same they should work fine in parallel feeding a single controller. Yes it is best to have same panels that match so if you have a good controller for the 250 consider leaving that in place.
