I've never used things like magic erasers for marks, I'm completely soap and water ONLY on my ponies - but I never had any issue shampooing Italian hair with a diluted mild shampoo (I think some people use washing up liquid/dish soap but I haven't tried that).
I don't think it's more fragile unless you can see any obvious damage to it (such as frizzing, or signs someone has used hairspray etc to the point sections have stuck together - which can make any pony hair brittle if left on for a really long long time).