The petunia seeds will be next to impossible to sow in pellets. The seed is so small. Unless you get the pelleted ones. I usually start all flowers in flats and them trasnplant them into peat pots for the same effect. The pellets work great for veggies like you said. I have used pellets for geraniums in the past. One thing about pellets is they dry out very fast due to how exposed they are.

I use Jiffy pellets to germinate all of my seeds, including petunias.

I get the seeds started in the pellets - usually aim for 3-5 seeds per pellet, depending, then when they are spouted and develop their first true leaves, I transfer them to larger growing pots/ trays (2" square pots, 20 to a tray).

Re: petunias. My weird trick to sowing petunias is use a needle. Yes, a needle. I put some seeds in my hand, then stick the needle in the moistened Jiffy pellet, a little of the peat will stick to the needle. Then, gently pick up a seed using the needle - the seed will stick to the moist peat, and carefully set it down on top the peat and gently press in.

Yes, it sounds wacky, but I've been doing it that way for, oh a decade - hey, it works! :)

I start all seeds in a good quality potting medium, never in the all-peat moss of jiffy pellets. I use plastic flats in which I place heating cables before I fill with medium. The seeds are sown in rows (or partial rows), and carefully labeled. Each variety is ready to transplant into cell packs (usually 2.5 inch per cell) at different times, making that process convenient.

Success rate is as close to 100 percent as it can be. One plastic flat usually yields about 500 or so seedlings, though it depends upon the size of the seed. I've been doing just a couple of flats in recent years.