You can't kill an Oleander - just start clipping. Keep in mind that every where you make a cut, TWO new branches will grow, so cut them back to the trunk to make a full shrub. It won't be long before the Oleander caterpillars defoliate it, that is to say, prune it for you!

Removal of the seeds will do no harm to the plant.

I grew up in San Pedro Calif with lots of Oleander to take care of. We always just pruned it when it looked like it needed it, wrong time of year or not, and it did just fine. Do you get frost or freezes in Redding? If so then I'd wait until after they hit to anything major on them, that way the outside branches protect the inside ones. But if your winter is more like ours is here in Arizona I trim a bit now, mostly dead scraggly parts but keep my trimming to a minimum in case this is the year we get hit heavy and hard on the freezes. It's kind of roulette with the weather in this part of AZ. We had only one real freeze last year, but the first year I lived here we actually got snow, 5 years ago. Just a few inches but enough to make the plants happy that we didn't cut them back more.

I live in Galveston, the Oleander City. They are everywhere here. And it is HOT and HUMID. Our humidity is rarely below 80%, we are an island after all. They don't have a tendency to get leggy here at all. There are tons trained as standards and even more that haven't been pruned since the last hurricane did it and are big bushes. It depends more on the hybrid then anything. Every few years I chop mine down to the ground, really, even with the soil almost, well, as short as I can get them without the chainsaw. They come back and are at least as big within 1-2 years and have always bloomed the year after pruning. Don't plant the thevetia if you want something bushy. Here every single one is a small tree, whether you want it to be or not. And they are a lot more toxic than the oleander. In India the seeds are chewed to commit suicide. I prune mine like crazy and it is now about 15' tall. And the seeds pop up everywhere. Though the hummers are really fond of it. And they grow in full sun. You'll get leggy plants and few flowers in shade. Depending on which one you get they can get up to 20' tall, hardy red for instance can be 20' tall and about as wide. The Oleander festival is next month here on the island. I'm in a 9b semi-tropical climate. Alkaline sand soil. Drought or monsoon. Tropical storms, hurricanes. They are pretty salt tolerant too growing right up to the edge of the bays/bayous. Tally HO!