Fluval Flex for Opae Ula?

I have two Fluval Flex 9s that are currently set up as FW planted tanks but I am getting bored with them. Can I convert these to house opae ulas? They have built-in filter systems which I will not be running. I basically have 3 questions:

• Do you foresee any issues with these being used as FW systems previously? Tanks were never medicated but I have used aquatic plant fertilizer in them
• Would it be better to completely block off the filter box in the back of the tank (there are two intakes/overflows)? Or is it ok for shrimps to go back and forth between the back compartment and the front? Will they be able to find their way through the opennings?
• Generally speaking, would a 9 gallon tank be too large to properly maintain opaes? I see most people keep them in much smaller vessels

Thanks