I cannot speak to your sounding questions quite yet, but as far as the second question goes:

As far as being "borderline dominant", this could mean a lot of things: that you show switch tendencies; that she perceives you to be topping from the bottom (trying to control the scene as the submissive), or perhaps that you aren't performing up to her standards as a submissive. Without more detailed info about what led her to make that statement, I can't really say.  

In BDSM culture there is a pervasive and damaging idea of "true dominants" and "true submissives". Under this paradigm, anyone who has switch tendencies is cast aside and led to believe they're somehow below people that *only* like playing as one or the other. It also creates false distinctions between different people of the same power orientation. "You won't disregard your limits and do things you're uncomfortable with? You're not a TRUE submissive!". "You're not 24/7 with your slave? You're not a TRUE dominant!", are some basic examples, but it runs much deeper than this and is inherently shaming of anyone not confirming to very strict (yet subjective) guidelines of what it means to be a submissive or dominant.

Plenty of dominatrices see clients who do not exclusively submit. I absolutely understand dommes requiring said clients only behave as submissives in session, but I do not agree with the idea of not playing with switches because they're versatile in their power orientation, or that 'borderline dominant' be seen as a disparaging comment about someone who is also interested in exploring submission. Please do not hold back from exploring all avenues of kink that appeal to you at risk of offending or losing a play partner.

I am a switch and I play as a switch in sessions sometimes :)

I see a lot of Doms (or those who are more dominant), but not always as a bottom. Many Doms have desires for certain activities that they would never ask their personal subs to do for them, so they come see me. I don't consider they are submitting to me as much as they are just bottoming for certain activities.

I don't require submission, as such...I require a respectful attitude and a willingness to play, things I get from both subs and Doms, and I love when I get another switch to play with!  

So long as you maintain your respectful attitude, and don't constantlly try to top from the bottom, I would think any Domme would be happy to see you. If you are concerned by her statement, then ask her, not us, and work with her to have the attitude she prefers.  

Now to your sounds question. Is she planning to use the wand ON the sound once she inserts it? I have tried that a couple of times and my guys don't seem to really notice the difference. You could gift her a vibrating sound to use on you, I hear those are lots of fun If she is just going to use the sound on you while you have sounds inserted, I would suggest you not wiggle around too much.

K

see my reviews of Mistress Mercy and Fiona, among others. An electrified sounds was inserted into my penis and a metal electric butt plug inserted into my anus. The effect was--electrifying. Seriously, quite astounding. She has to be very, very talented, though--no amateurishness.

...is that it can take so many different types of attachments My sounds don't fit with mine (I have rigged them to one of my TENS) but given the right size wand and the right size sounds or adapters, it could work.  

As it's your first time with sounds, I would politely request her to be merciful and begin with small rosebud sounds.

As for being borderline dominant? As long as you know your place when you're with her, then what would it matter if you chose to take a dominant role with someone else? I can't imagine that being a problem, as long as you're not trying to top from the bottom with her. Some people love bratty subs... not me, but some people do.

I am a pro and lifestyle domme and I have both sounded many people and been sounded myself. With sounding, it's important to maintain a sterile working field (bzk wipes applied to the area and sound if it not in a sterile container plus surgilube packets.) Sounding can be painful or incredibly enjoyable depending on your particular anatomy. As far as the violet wand goes, most sounds are not made of metal that conducts the electricity from the violet wand.

To prepare yourself, just relax, stay still and remember that she will not do something to harm you and in the event that that happens, the potential damage with sounding is just a little burning when you pee afterwards or maybe a UTI (unlikely.)

Personally, I see many men who are dominant in their every day living and it is not a problem for me because we both understand that they are giving their submission to ME; Everyone is somewhere different on the scale of dominant and submissive and that scale often changes depending on who you're interacting with. For example, you might be dominant over a girlfriend but not dominant over your mother. So it sounds to me like the issue is more that she's worried about you being unaware of your own orientation or she may be concerned that you may be more dominant than her.  
Solution: Focus on giving your submission to her or if you don't feel like you can do that and you still want to be submissive it might be worth while to find a more dominant woman.