I used Flourish Excel once a week in a 5 gallon tank dosed as directed with no bad effects. But yes, I have heard of people who deliberately overdose excel harming shrimp by accident. But in the same breath, I've heard people does up to 2x the normal amount and be ok. Just keep up on WC's as to not let anything build up over time. Dry ferts are always good too.
You guys are killing me with all this shrimp talk. I've got a 2.5 gallon that just begs for some shrimp!