I'm really surprised to see people saying you need an order to piggyback something. I have never seen an order for anything other than "IV." You can dilute things as much as you want as long as it meets the minimum as specified in the policy or instructions from pharmacy. For example if policy is to reconstitute a gram of Vanco with 20 ml SW and you only mix with 5 ml, that's where you're going to run in to problems.
In my facility, nurses on the floor are not allowed to push anything. They must use syringe pumps or piggyback. Nurses in critical care areas (ED, ICU, PACU) however can push everything that RNs are allowed to push per the BON.
Some nurses still use syringe pumps in my department (ED), but I prefer to push almost everything that can be pushed. Of course things like Abx need to go in the syringe pump, but narcotics, lasix, etc all get pushed.
You need to be familiar with your facility's policies. For example one place I work says up to 10 mg Morphine needs to be pushed over a minimum of 2 minutes and the other place says 10 mg should take at least 4-5 minutes.
If my pt has never had IV narcotics before or if it's an elderly person, I will push the first 1/4-1/2 of the dose extra slow to make sure they're tolerating it ok then will push the rest at the speed suggested by pharmacy/policy.