[@Angel Vicky] I love questions!

1. Once you guys get partnered up, you can decide with your partner what your new mark looks like and where on your bodies it goes. Keep in mind though that they'll always appear as mirror images of each other (ex. if your mark is on your right arm, your partner's will be on their left). 

2. There's never been one recorded, and for the sake of simplicity I'm going to keep this RP to two-member pairs. 

3. Soul bonds don't change, but it's entirely possible for a pair to misunderstand their bond. For example, maybe a pair can't stand each other and thinks for sure their bond is antagonistic, but start developing romantic feelings for each other anyway, and eventually figure out that it was a romantic bond the whole time. Or maybe you have two siblings who are otherwise close but share an antagonistic soul bond that makes them endlessly competitive with each other! You can really have a lot of fun with soul bond manifestations, and I welcome discussion with both your partner and with me if you want to see how your ideas might work ^_^

4. Romantic pairs will always be compatible sexuality-wise, but that doesn't mean you can't be confused at first. Similar to (3), it could be possible that, say, someone who went through their entire life considering themselves gay discovers some time after they find their soulmate that they might actually be bisexual and didn't realize. (it's important to note that their sexuality isn't changing, it's more like they're finding out they may have wrongly categorized themselves in the past, as some of us do). For this purpose, if you want your character to possibly undergo something like this, shoot me a PM when you make your character so I can take that into account! 

I hope that answers your questions! Feel free to ask more as they strike you ^_^