Different strokes for different folks really. I usually use 10ml of solution when I have rectally administered pills. I have this notion in my head that slightly more solution exposes more of the drugs to a greater surface area more quickly. I really have no idea if this is accurate, though I'll say that 10ml of solution is easily tolerable for most people.
If I were you i would crush the pill, mix with ~5ml of warm water and try that out. This should be enough to completely solute the pill and make the solution homogenous.
I'm a pretty big proponent of this route of administration. It occupies a good middle ground in terms of safety and potency, I always felt that Opioids and for that matter, other drugs administered rectally had more of a "physical" feeling than say, insufflating would. It kind of feels like the feelings are emanating from that area near the bottom of your spine and radiating outwards.
It should be a good experience. Write us a trip report why not? There aren't a lot of TR's featuring Hydromorphone via this ROA.