One, it's venom, not poison. Poinson must be ingested (swallowed). Venom is injected (like a snake fang or stingray barb).
FW rays actually carry a complex protein that is more toxic to humans than most saltwater rays. Also, a barb puncture doesn't always mean venom injection. It's very similar to a hot snake. Venom is manufactored by a small galnd. A hot snake can bite you and NOT inject - same as a ray can spine you, but not hit you with the venom.
As for what it feels like when you do take a shot of the venom - I've previously described it this way:
Take a fork and stick it in your arm. Then in that openning, inject gasoline. Wait a minute for that fuel to soak in, then light it on fire. I've taken dry shots and had the ray rip the barb out. I've also take a few hits of venom and wanted nothing more than for someone to remove that part of my body to make it stop hurting.
Aqs for hitting a tankmate - I've had them get catfish before with both positive and negative outcomes. Often times a secondary infection sets in at the point of penetration.
Here are two shots for you to check out. The first is a sample of barbs from various motoro, leos, hens etc.... (all freshwater) next to my hand for size comparison. The second is a close up of the edge. You can see why you don't want to pull them out backwards.
