Toni,
The usual practice is to use a balanced fertilizer at all growing stages. I don't show plants but I understand that those who show use a blossom booster to promote heavy bloom just before a show. It's my understanding that using it always will tend to cause a crown die-off.
As far as promoting leaves, while we all want blooms, they are not vital to a violet's continued life and growth. Leaves are. They are the food factories without which we have no blooms. Healthy, well fed leaves will give us great blooms, not vice-versa. No leaves, no plant.
I understand that Jolly Jubilee is a difficult bloomer. I just bought one and I can understand why. There's very little chlorophyll in those leaves. I am going to try and green mine up a bit to see if it helps with blooming.
I treat my minis the same as my standards as far as care and fertilizing. I do watch them a little more closely as to watering. In my experience, the smaller the plant, the smaller and more delicate the root system and the more susceptible to rot and/or dessication.
If there's an age and/or size violets must obtain to bloom, I've never figured it out. Much depends on growing conditions, (better conditions promote faster, more robust growth), and the variety, (some varieties are go-getters, some are not). Plants bloom when the root system and top growth is strong and viable enough to take on the stress of blooming. The rule of thumb is six months from leaf-set to bloom but again, many things influence this.

Linda