Continuous fentanyl infusion and othe meds

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Hello, I've come across this question in a couple of different sites I've worked at and have never received a definitive/scientific answer that satisfied me fully so...here goes.

I have a micro-prem on continuous fentanyl infusion of 2.5mcg/kg/hr that is infusing with his primene. We have been using a peripheral IV for his multiple meds (2 antibiotics and 1 anticonvulsant at this time). He has had 4 IV starts yesterday and he is running out of sites. I've asked why not infuse the meds with the primene and fentanyl. All the meds are compatible with fentanyl but I was told that the concern is that the baby will get a bolus does of fentanyl with the administration of other meds. In the past, I've been told that it really doesn't matter because the IV catheter is like a river and it has a set rate and that by adding a medication does not affect this rate. In other words, by adding vancomycin to the existing fentanyl and primene does not affect how fast these drugs are being delivered.

Can someone clarify this for me? TIA