Any of the wave petunias work really well for containers or hanging baskets. I am going to be starting some this year too. I just bought a variety of easy wave, tidal waves and shockwave petunias from Ebay (make sure you get pelleted seeds). I grew them last year and they turned out wonderful.

It's strange but I had always heard that petunias were hard to grow from seed but then had some volunteers spring up in my zone 5 upstate NY yard. So abouty 2 years ago I saved seed then wintersowed and also did regular seed starting. It was so easy and I ended up with too many plants! The problem is they take a long time. In your zone for bloom before July or Aug, you'll have to start them early indoors. Wintersowing worked really well and was easy but took a while to bloom.
I snow thickly (since seed was free and plentiful) in vermiculite not covering seed but plastic bagging then transplant clumps to 4 or 6 compartment things in flats when they have a few leaves

Here in Zone 5, I start my petunias anytime between March 1 to March 21; you should get bloom by mid June or so. I'm doing my final transplant in the ground, though. Might be quicker bloom when in a planter with the limited root space.

Also - what Cathy said above. I'll add that you might try transplanting some individual seedlings into planting cells or small cups. They grow faster without the competition of others around them. That being said, I often do grow some tiny seeded plants like the petunias and annual lobelia in clumps, and they do just fine.

Tropic Lover - your plants are GORGEOUS!

- Alex