Will Rubio alone give me a good seal and finish?
These are really two separate things. The finish is about aesthetics — how do you think it makes the wood look? So nobody can answer that for you. You've indicated you're planning on testing the finish out on a scrap or two and you'll see then1.
Now as to the sealing aspect, Rubio would have consumers believe that it does in fact seal the wood really well in one shot, but user tests have confirmed that it is far from bulletproof (which is no real surprise given the thickness of the finish on the wood).
However, that said Monocoat is more than good enough for a typical bed2. You're not finishing a tabletop after all, this is much more akin to table legs which are typically more lightly finished than the top because they don't need to withstand the same rigours.
Or should I pre-treat the wood with anything to close up the pores in the wood?
I'm sure you can yes, Monocoat must bond reasonably well not just to wood but to any wood filler used to plug nail holes or other defects, epoxy fills in cracks and so forth, natural resin pockets in the wood, or its use would be pretty limited.
Whether you should is again a judgement call, but ask yourself what you hope to gain by doing so and whether it's worth it for a bed, even merely on something of this size and shape? It is a ton of extra work on something sizeable and very difficult on anything with inside corners, or anywhere wood joins wood at 90°.
I'll likely end up sealing a piece of scrap with it to get started
This is the right call. It's always a good idea to test out a finishing regimen before committing to the workpiece. Always, even when you've used the finish before. But 100% of the time when you're using a finish new to you.
but the stuff is expensive and I don't want to waste any if possible.
Remember you can always test finish options on the back sides or undersides of pieces of a project, rather than on offcuts.
It may give those hidden surfaces a harlequin appearance if you go through a number of options, but normally nobody will see them so it doesn't matter.
1 Remember that finish tests should exactly mimic the process used on the project, so the test pieces must be planed, scraped or sanded to the same level as the project wood for the text to give a proper indication. Just sanding to a lower grit (e.g. 150 instead of 220) is enough to give a radically different look in finishing.
2 I want to be clear this is not a plug for Monocoat, the same is true of just about any oil or oil+wax finishes.