Great! Fun, fun, fun.
I use Osmocote-Plus and give them about half-a-dozen prills each.
I don't think the form or brand is important; just the management of nutrient minerals matters. Osmocote prills release more fert the warmer it is, roughly matching plant needs, so I'm free to day-dream. If you already have a fert regime, just use that and adjust your schedule accordingly (clearly you are aware of timing for bonsai).
These babies get repotted about every two years. Basically, when it gets blown off the bench is how they get new pots. In fact, the little cup of tea was such a solid ball of roots that it may have had something to do with why the demitasse broke. I'm looking into whether being root bound makes the needles shorter than just being small does. I have a number of bigger ones, too. I repot them about every 2 years now - some in spring before the buds break and the others sometime in summer or early fall (Aug/Sep typically).