Mysterious green lines in space

PoSes are safe in HS. They cannot be attacked without an active war, or the attacker will get CONCORDed. It is not completely safe, but if you see a notification for a war pop up, run over and put more hardeners/ECM on it. Don't bother trying to have the PoS attack back - you want to make it as annoying and difficult as possible for attackers to damage your PoS. Also remember to load it to the gills with Strontium, so that the reinforce timer will kick in.

The only things you can't put on a PoS in hisec are reactors and moon miners, IIRC. ( and of course silly things like jump bridges and capital things ).

The maximum level of security you can put a PoS into is 0.7, no matter how high your corp's faction standing is. The difference between 0.7, 0.6 and 0.5 is minimal - the biggest thing is the response time of CONCORD, something a PoS isn't terribly concerned with.

Inside a POS bubble, a ship will stay in place indefinitely. Outside of a POS bubble, rookie ships and shuttles will stay for 30 days, and other ship types will stay indefinitely. Of course, by indefinitely I mean until someone finds it.