No - you can't use 'all rates fixed' @ 24db. The SR card max at 54mbps is 17 (If I remember correctly). @ 48 it's 19db, @36 I think it's 22db and the rest are 26db. 26db is 400mw.... So 250mw would be close to 24db....
And - right now unless the latest release of ROS has changed this behavior - you have to set the card to 10 less than you want, i.e. you want 24db out then set the power at that data rate to 14db.
I am not sure where you are but if you are in the US then you can go above 250mw on a Point-to-Point link at the input of the antenna connector. I don't remember the exact power level but 400mw keeps popping in to my head and I don't remember if this was the PtMP or the PtP level.
Do you have enough resorces to try that link at 5ghz?
This quote from Equs - he was asking you if you had sufficent resources to swap your backhauls out to 5Ghz instead of using two 2.4ghz channels.... Thus keeping a little more spectrum for your own use.... Because using two 2.4ghz channels is always a challenge when they are (physically) close together. And since you are using two 2.4ghz channels - hopefully you are using 1 & 11 at the sites - a larger frequency spread between the two channels will give you a better chance at maximizing your throughput. Remeber there are only three non-overlapping channels 1, 6, and 11, 2412, 2442 and 2462.........any other selection will overlap and cause you issues.
So back to you original question - no you don't set the power out to 'all fixed rates', instead you'll have go look up the SR card specs, and set the power you want / and or can achieve minus 10 for each data rate you want to use.
Lastly - you can set the RB to B/G mode - the AP will connect to a client at whatever rate / protocol (B or G) that it can support and the data speed that it can connect at.....
Good luck out there!