I have kept a few cows and boxfish and the only time I have ever had an issue with their toxin was while transporting one during a power outage. Unless they are with a fish that is frequently nippy with them, I wouldn't be too worried about their toxin.
They don't do well in high direct flow, but can handle indirect flow well.
It is difficult to tell if they aren't getting enough to eat, so overfeeding and noncompetitive tankmates are a must.
Cowfish and boxfish are especially susceptible to ich, but if are not bothered by tankmates and are well fed are quite resilient.
They will nip at fleshy LPS.
Fish that I have kept with them include: copperband butterfly, multibarred and golden angels, leopard wrasses, pipefish, Paraclinus blennies, barnacle blennies, orange spot blennies, starry blennies, Meiacanthus species of blenny, flame and longnose hawkfish, jawfish, most gobies, pipefish, firefish, wormfish, royal gramma, blackcap basslet, swales basslet, elongatus dottyback.