Almost all the guys and even some of the girls can be romanced by unlocking their side stories and maxing out their favor levels. Seeing as I've already maxed out Alan and Barris you should be able to be Mr/Ms steal your girl/guy and romance multiple people at the same time.

As for the picking out clothes, I'd say use the trade in section in the gifts menu to trade in for 3-4 star clothes of colors you don't have. What I did when I just started out was to either have friends whose avatar had the color of clothing I needed or just avoid that ball all together and resetting (entering and exiting the outings menu/closing and reopening the app) till you get the color you are strong at, works very well for maxing out specific characters too.

But if you really have to go to the ball for story purposes, take note of this. It's not how pretty your outfit looks, it's how many things you spam that matters. Since every piece of clothing that fits the tag will earn you chic points, try to prioritize that over most clothes. However, if you have a 1~2 star clothing, it might often be better to just forgo it for something of a higher rank.

Often times if I'll have to choose between just a dress, or top and bottom. If you have the latter that fits the tags, you should go for it(assuming the rank on the clothing is at least 3~4 stars of course) Since there's no difference in chic points, the latter is the better option because you can get more variety on the elements eg. lively, sexy etc.

Note that stuff like accessories, handheld, special and bracelets can stack so just wear as much as you can. Special is also quite good as often times one of them gives you like 30~40 chic points so those are definitely good and I'd try to get my hands on them asap.

Sorry if this is way too long but I hope this answers your question and that anyone who reads it can get better at the game. PS: feel free to ask me anything else about the game