I would say, if you know your scales, pents, whatever well, stay in standard 1st, and use the D/G/B strings for your major chords with the slide. on those 3 strings from open you get:
G/Ab/A/Bb, B and so on, like A shaped bar chords up the neck.
Next, find the Phyrigian(hope I got that right) box (yes, pents can be Phryrgian too!)for the key you are in, it is a great place to start
The problem you will have is sliding and getting the slide on and off the strings. Slide is more about sound and feel, then dead on accuracy. Damping is very important and may force you to learn some new techniques. I play can with Fingers, Pick&Fingers and Pick only, they are all different- but many great sliders use all or some of these.
Play with a heavier slide, and let it do the work for you. Play over the frets, listen to be in tune.
Put the slide on any finger that is comfortable for you, there really is no way to do this wrong. It will sound like **** at first, overtones and stuff, and intonation being off. Just keep at it, it will payoff.
Joe plays in a few different tunings, E-G- and D, and I think I have heard like an Em or DADGAD tuning in there too (He is after all a big Zepplin fan)
I play about 8 different tunings, slide is kinda my thing. I make bottlenecks, and also do the Sonny Landreth behind the slide thing. I will gladly answer any questions you have about playing slide. I also play lapsteel. Would love to learn pedal steel but I am dyslexic and learning that would require faculties I don't have
You may have too (to get a good tone) raise the action a little on a guitar. Most people make the mistake of jacking it up, which actually makes it harder to learn. raise the action in quarter turns, until you can play both slide cleanly, and still fret when you need too.
Open G is closest to standard- only dropping 3 strings: DGDGBD (low G) the advantage to this tuning is all your blues pents on the DG and B remain the same, the rest you have to adjust upward 2 frets, but they are close!
Open G is primarily a Country Blues tuning but I love it and use it for everything, minor, major, 7th blues, country, rock- EVERYTHING---- the secret behind the Stones sound (Keef Richards) is open G. Ry Cooder taught it to him (and rumored that Ry also did Honky Tonk Woman on record, not Keith, and was not given credit, causing a lifelong battle)
Warren Haynes does play in standard, because of Gov't Mule, but he prefers open E, big Duane fan. Open E is tighter on the strings, and some people like that sound (open A is the same way) Billy Gibbons is a big open E guy, but usually just plays in Standard
Bonnie Raitt was taught Open A by Lowell George of Little Feat.
Dave Hole plays over the neck with the slide on his 1st finger and uses a pick, totally backward from conventional technique.
So, play on , spend some time with it and learn to work it