1)sea horse tanks requires a chiller below 28 should be good enough
2)you can mix them with corals and stuff but as for fishes do not mix them with aggressive fishes as seahorses are slow and they will always lose out
3)you can always start off with 1 its alright but some feel a pair is nice
4)nope colour change doesnt mean its sick just adapting to the area also beware of scalps on the seahorse body