Regarding so-called 3 currencies, and if we are talking about the worth of each of them you should know that STEEM = Steem Power.
In other words, you can always Power Up your Steem Power with STEEM and that conversion will be done in an instant, and the ratio is always 1:1.
The process works the other way as well, and it's called Power-Down but with one big difference. When turning (transferring) Steem Power into the STEEM (no matter which amount is in question) for the Total in holding will take about 3 months to be transferred into the STEEM. Meaning, approximately 10% per week, where the first amount would arrive in your STEEM Wallet 7 days after you have requested the Power-Down.
Steem Dollar (SBD) works like a separate currency (with its own rates) although it's somehow connected with the STEEM, but as far as I understood the intention of creators was to keep it always around $1 US.
The other thing that might be important regarding STEEM and Steem Power is different usage purpose, while STEEM functions like any other cryptocurrency out there, Steem Power is the one that gives your votes certain value. As well, the amount held in the Steem Power brings you particular interest (to cover the inflation of new blocks being created), while the same amount held in the Wallet as STEEM wouldn't collect that interest.
Regarding the slider itself - yes, there is a good reason why it's not available until the member reaches 500 SP, and in that sense, I was even sorry afterward that I published about this other option through chainBB.com as many users didn't even close master how to handle their voting power and keep it on the optimum level daily, what I believe they are forced to learn without the Slider. Not to mention that votes at a very low percentage, and when the account is below 500 SP wouldn't bring any benefit as not for the member to whom the vote was given, even less to the curator (vote giver) or even could not be of the accountable amount.
Hope this would give you some more answers as far as I could read your question in-between the lines. 😉😊