Well you will definitely want to plant your groudncovers, shrubs and companion plants before the trees roots extend into that area. Do it in the fall of course, don't attempt planting now. I'm a big fan of the low growing junipers for groundcovers. Some will say 'oh thats so boring zzz zz' but the blue rug makes a nice ground cover for the foreground and will reduce the amount of mulch you'll need. The slightly taller ones like parsoni and grey owl provide seasonal intersest from the year round color, the pleasant fragrance and they are really trouble free. They grow under trees and in full sun too, very drought tolerant. There's so many varieties, shapes, sizes and colors. As I mentioned before I would not recommend planting annuals in your peanut bed. Make a separate flower bed for annuals. MAke your peanut bed a permanent fixture in the landscape that won't require constant maintenance, digging and fluctuating regimes of shallow watering. I would put something like sea green juniper in the back, some perrenials in front of that and in front of that, a hedge of soft touch hollies and in front of that some blue rug. Beside the perennials, a specimen like a viburnum or something else that stands out. You'll have a nice low maintenance peanut bed in no time.

The only thing that is important is that the azalea gets enough shade. They like partial shade in hot climates. Some varieties need more shade and some varieties will not grow in a hot climate. They get a disease that kills the roots. First you should find out how much sun the variety you are planting needs. Most like partial shade. They need some sun to set flower buds, but some shade to keep the roots from cooking. Mulch is also very important to keep the roots cool. To increase your chances of success, make sure you have a well drained bed. Sandy soil is very well drained. If the soil is not well drained, plant in a raised bed to insure good drainage. Here is a link that might be useful: How to care for rhododendrons and azaleas.

Why remove the leaves? The plants love them, but I know if you're dealing with an HOA, you may not have a choice. I had bromeliads, gingers, amaryllis, crinums and philodendrons under my oaks. I planted the philodendrons by laying them up against the oak tree trunk and covering with a pile of dirt. They eventually grew up into the tree and were gorgeous. I loved not having to rake under the tree, or even fertilize much, because of the leaves. Never "planted" a bromeliad. Just laid them on the ground and let them do their thing. The gingers only need an inch of dirt over them to grow. The giant crinum was planted quite by accident when I was given one which I tossed under the tree and forgot about. It rooted into the ground, still growing on its side, and bloomed like mad every year. The amaryllis I started as tiny bulbets given to me by a friend, so I only had to dig a tiny little hole and cover with dirt to start them. They took a few years to bloom, but they did, and multiplied like mad. There is a grass that will grow under oaks. It's called "Bitter Blue" St. Augustine, and it's not that easy to find. Mine was there when I moved in, and wherever it grew out from under the shade, it died, because I didn't irrigate the lawn. Still, it thrived under the oak. Monkey grass is also easy to plant and grow under oaks, because you can pull the clumps apart and just plant the sprigs. It fills in rapidly. Same with the green or variegated ornamental sweet potato. Just root some cuttings in water and plant them as soon as they have tiny 1/2" roots on them. Of course, you can always mulch and use containers for flowers if you want. A lot of people do that.

I agree that (some) ferns are as tough as old boots. Also, they don't necessarily require damp conditions. I have planted a number of ferns - basically whatever K-Mart was selling as Baby Ferns - along the narrow passageway between fence and house. Although they are protected from our heavy frosts, they still have to contend with the winter cold of the Canberra region. Some turned up their toes in the first winter, the survivors went on to endure our drought. Over the past 3 years, the only water they have received is the odd shower of rain. The oak tree, at 100 years, should be providing high shade - that is, shade from direct sun but still allowing for a lot of light underneath. There are oodles of plants which would enjoy the environment, including shrubs. I would definitely recommend the old acanthus (oyster plant or bear's britches). After dying back after flowering in summer, it shoots up again in autumn. The large dark green leaves are quite impervious to cold and would provide a luxurious green under the bare oak. If it were my garden, I would plant lots - they grow from root cuttings. The leaves themselves look fantastic edged with a rim of frost. Lots of photos on the net of the flowers, but finally found this one of the plant. Here is a link that might be useful: http://www.hgtv.com/hgtv/gl_plants_perennials/article/0,1785,HGTV_3610_1956068,00.html