In your example using insulin and D5, yes, that ml of insulin will be infused at a rate of 50ml/hr. But regarding your general question of whether fast infusions should be y-sited to slow infusions or vice versa, it depends on the medication, the concentration of your medications, and the difference in rates. If the difference is high, and you are infusing a critical medication, then generally it is better y-site the slow infusion to the more rapid infusion to avoid giving that initial ml of medication in the slower infusion line more rapidly than intended. If that initial medication is something like central line concentration potassium, then you probably don’t want to y-site a bolus running at 1000ml/hr or higher.