Q: I don't really want to buy a gown now, because at my age (over 70) I have very little use for a formal. However, I am to be seated at the main table at a relative's wedding dinner, and I want to look presentable. Can you please suggest something that would be really nice but won't just hang in my closet afterward? I'm 5 feet tall and weigh 115 pounds.--M. F. A: If you invest in separates instead of a one-piece gown, you will be able to wear the independent pieces with other items already in your wardrobe. For example, after the wedding, you can shorten the black silk crepe de Chine skirt illustrated here and wear it for dressy, but not formal, occasions. This skirt, priced at $45, would end at the ankles on someone 5 feet, 5 inches tall, so it should be full-length on you. It has six gores, an elasticized waist and two side pockets. It comes in sizes small (4-6), medium (8-12) and large (14-16). Write to Royal Silk, Royal Silk Plaza, 45 E. Madison Ave., Clifton, N. J. 07011. The skirt also comes in navy, gray, red, hunter green and umber. The tunic top, priced at $24.95, is made of a polyester metallic knit available in both gold and silver, in misses' sizes 4-18 and women's sizes 36-46. If you think that the V neckline is too low, fill it in with jewelry or a gold lame scarf. The tunic is featured in a catalogue published by Old Pueblo Traders, Palo Verde at 33rd streets, P. O. Box 27800, Tucson, Ariz. 85726. Credit-card orders can be telephoned in to (602) 747-5000. Q: What length gloves should be worn with a sleeveless bridesmaid gown?--P.S. A: The "Bride's Book of Etiquette," by the editors of Bride's magazine, states: "The bridesmaids in a very formal wedding almost always wear gloves to complement short or sleeveless looks." Lace gloves made of nylon spandex are available for $13.50 in the spring-summer '86 J. C. Penney catalogue, Page 163. Q: Is it proper for the bridegroom to wear tails and the best man and ushers to wear tuxedos?--A. P. A: The same authorities quoted above say it's OK. They even approve of the bridegroom wearing a double-breasted tuxedo to walk down the aisle, with the best man and ushers in single-breasted tuxedos. Tails are appropriate only for formal weddings.