I have a few suggestions below. You may want to present these to your attendants and see which ones they like best. Any combination will work. You've chosen a monochromatic color scheme so any lightness or darkness of purple is acceptable.
Dress your bridesmaids in lavender and your Maid/Matron of honor in plum. I did this at my wedding. Our colors were silver and light blue. I wore a white dress with silver and light blue accents. My bridesmaids were in light blue while my Maid and Matron of honor were in a dark silver. You could also, conversely, put your bridesmaids in plum and the maid/matron in lavender. However, it is generally more popular to dress the maid/matron in a darker color so as to contrast the bride better.
Dress them all in lavender with plum sashes or accents. The accents may be shawls, wraps, or ribbons. Many brides who choose this route also have ribbons of the accents in their bouquets and the attendants' bouquets.
Dress them in plum dresses with lavender bouquets. You may also want to include lavender accents. Since the lavender is on your dress, you definitely want it to be more of a secondary color than the plum. So if you do dress them all in plum, you need to make sure the lavender definitely stands out as well.
I advise against dressing them in patterned dresses or dual-tone dresses. This will make your wedding party look too busy since your dress already has a floral pattern. Stick with elegant but non-ornate dresses for your attendants. I personally would go with either the 1st or 2nd suggestion.
Taffeta is rarely found in bridesmaid dresses , this is because it's a very luxurious fabric and to be honest, it's likely to stand out more than the bride!
You're looking more at high street retailers which only stock 'fashion' colors. You'll be paying a little extra but it's worth going to bridal stores as they will have the color selections all year round. They usually have all styles available in all colors too, so you should find something you like.
Mori Lee has a stunning chocolate brown which is available now, have a look on their website for the different styles. Forever yours Romantic also have chocolate brown in their collection. Most designers have bridesmaid ranges these days.
All the designers have stockiest lists on their websites so you can find the nearest store to you. You'll usually find galleries as well so you can have a browse at some of the different styles they have.
Plum silver sounds lovely! Silver accents would be lovely on your plum dresses. If you go for another color, it can over-complicate the look. As silver is the 'accessory' color (this means its the color used to complement your main color, which is used in hints) then it would be very suitable.
What you have suggested sounds lovely! I can especially picture these dresses in chiffon. Organza can be quite stiff, the cheaper organza has a shimmer to it and it makes it look cheap!
Despite it being the evening, it'll still be light so the tangerine sounds like s good compromise. Very zesty!
Do be warned - please try this color and T length style on your bridesmaids as it doesn't suit everyone.
Your bridesmaids would really need to be tall for the length as it can stunt shorter ladies.
Tangerine is a lovely color but can wash out some complexions - also if any of your bridesmaids are larger they may well end up looking literally like a tangerine!
If tangerine is too severe then try a more peachy orange instead. It's softer and kinder to complexions and will look very elegant in chiffon. Lemon or sunshine yellow is a lovely color as well, believe it or not. Along with a golden tan (fake or otherwise!) it simply glows.
Long, flowing drapes and gentle scallop frills are very much in this year and will look super on your bridesmaids. If you have larger bridesmaids then the styles with the folds / roaches across the waist draw the waist in and make for a shapely figure. There is some lovely t-length styles lovely peachy color. This color is really lovely and is one of the colors I'd recommend as an alternative to tangerine.