You can add in a LOT more to the 90 gal. YOU have haps and Labs mixed and they do have a habit of cross breeding so you may want to make sure on the sexes and change some out. Most speices will not spawn with another speices, but males are so persitant.. Just remember that some, the Haps, get to 8" at adult.
On your plecos, they don't like a lot of salt. SO you may only want to do Half doses..I have no plecos in my African tanks..some africans also need the GREEN algae that the plecos eat..Plecos dont eat the brown/red algae you are going to have also, so you are stil going to have to scrub the rocks down as it grows OR it wil be hard to get off in the long run..FYI
I have found that a bucket with the TOP rocks and POND algae killer(FOR PLANTS AND FISH) for 4 days wil remove the stuborn algae..Scrub lightly and rinse with a hose..My dolomite looks nice and white again..Bleach is not recommended due to the softness of the rocks may absorb the bleach.. That you don't want.
This is before I cleaned the rocks:
This is after:
