Hi Workout,
I'm going to start with your last question.
The size of the bead cap usually doesn't match the size of the bead. That would make life too simple!
We have an 8mm bead cap that fit 12mm beads and up.
We have an 8mm bead cap that fits 8-10mm beads.
We have a 7mm bead cap that fits 6-8mm beads best.
Generally, I think you want a bead cap about 2mm smaller than the bead you're putting it on. But this depends on the shape and depth of the bead cap.
You can use just about any size toggle. I think the very largest ones are too heavy-looking for a bracelet, but that's just my opinion.
Spacer beads: As long as your beads are larger than the 4mm hole, you'll do fine.
On any given project, do try to use beads that all have a similiar size hole. Otherwise, the mismatched hole sizes make the various beads hang differently.
With the sterling silver 5.6mm alphabet cubes, I would tend to use the #26-505 6x5mm rounded sterling bead (with 3mm hole) as spacers.
If you want to use any birthstone beads as spacers around names, I would use the #26-591-8 8mm Stardust/Rhinestone beads (sometimes called Fireball beads?). They're kind of pricey, but a good way to tuck a birthstone into a Mother's Bracelet.
And then the #39-554 Sterling Silver Heart Toggle clasp would be nice finishing touch.
Don't forget, when using toggle clasps... the last few beads near the clasp should be fairly small, so that the bar can be pulled through the loop without a big struggle (especially important for bracelets, which you normally have to clasp with one hand).
--Polly