LONG POST! TL;DR at the end!
The video above is for those who don't want to read my wall of text below. This is a write up of my experience fitting standard pick-ups into a multiscale guitar and I wanted to share the experience with everyone in case someone is interested in doing this as well. That way if they run into similar issues they can use this as a reference to troubleshoot and fix an issue if it were to arise.
I recently changed the pick-ups on my seven-string guitar from Kiesel lithiums to Dimarzio Ionizers and really liked the sound of those pick-ups so I wanted to put those pick-ups into my 8 string multiscale guitar as well. Unfortunately, those pick-ups only came as a standard set online but after doing some research I found out you could take the baseplates off the angled set of pick-ups and then replace the baseplate with your new set of pick-ups by unsoldering the ground connections from each set of pick-ups and resoldering the new pickups to the old angled baseplate. I was super curious about trying this out and ended up buying a brand new set of 8 string Ionizers to try this with knowing that I could potentially be out that money if this didn't work.
The pick-ups arrived and I sat my guitar down with everything at my work table. I took the strings off my guitar and unscrewed the pick-ups from the guitar. Unsoldered the bridge pick-up and freed it from the guitar. Unscrewed the screws off the bottom of the Kiesel lithium pick-up which freed the bobbins from the baseplate except for the ground which needed to be unsoldered completely separate it from the baseplate. After that, I unscrewed the screws from the bottom of the Ionizer bridge pick-up which leads to the first issue I ran into.
The first issue wasn't a big one, there was a glue/epoxy holding the pick-up bobbins to the baseplate which I wasn't expecting due to the Kiesel pick-up not having this when I unscrewed it. I found out soon after the glue could be melted away with a hairdryer so I was able to free the pick-up bobbins soon after.
After that, I unsoldered the ground from both pick-ups and resoldered the Dimarzio pick-ups to the Kiesel angled baseplate. Then resoldered all my connections and plugged in my guitar to my rig which starts off the main issue I got. A ringing "line-in" kinda noise when I had the pick-up on any position besides neck or bridge.
I figured I must've soldered the ground wrong so I resoldered that connection and still got the same noise. Then thought I must've resoldered one of the other connections wrong so I resoldered all the other connections again and still got the same noise. At this point I was trying to think I was I did differently compared to soldering the pick-ups into my seven-string and thought the baseplate swap was the only difference. I figured that had to be the issue so I resoldering that connection and was greeted with the same ringing sound.
At this point, I didn't know what to do so I thought I'd reach to someone who probably does. I shot a message to the pick-up guru @KnightBrolaire (check out his pick-up threads if you haven't) and explained my situation to him. He gave me a few ideas on what it could possibly be, one of which was repotting the pick-ups in the case some of the microphonics were picking up interference. After a quick google search, I learned how to pot a pickup with some unscented candles I picked up from Kroger. I melted all the candles down into a glass jar and then dipped the pick-up in for about 20 minutes and then let it cool for a bit. Reinstalled the pick-up into my guitar and then plugged it back to my rig to get the same noise again.
At this point, I just kinda figured I was a loss and probably just needed to buy new pick-ups all together from a company that would do custom baseplates but I wasn't done trying to make this work just yet. I did some research on electricity and some apartments might have "dirty" electricity which could cause some issues so I picked a Furman power strip to get a cleaner source of electricity and just in case a power surge ever occurred. I thought this has to be the solution but alas I still got the same sound.
The last solution I discounted initially from KnightBrolaire was interference from electronics nearby which I didn't think would be the case, but this is what turned out to be the issue. When I had the guitar near my computer and facing it the sound would be produced but after turning it slightly to the side or just backing up a bit the sound would go away. I felt like a fool after going through all of those troubleshoots to fix an issue that could've been easily solved if it wasn't for my arrogance.
That being said if I did solve the issue from the start I wouldn't have learned as much about guitar pick-ups and electronics as did through my mistakes. I learned so much through this and even though I thought I was going to be out of the money I spent for the pick-ups I was super stoked on just how much I learned. Potting a pick-up, swapping baseplates, and electronic interference were all new concepts to me and I feel pretty accomplished by getting the DiMarzio pick-ups to finally work in my guitar without having to spend more money on custom pickups. Closing thoughts, fitting standard pick-ups into a multiscale guitar is really just as easy as swapping the baseplates just don't be stupid like me and discount other outside sources that could interfere. Shout out to the community, especially @KnightBrolaire , for helping me with issues I ran into. I hope this post helps anyone who might be thinking about fitting pick-ups into their multiscale guitar.
TL;DR - I successfully installed standard pickups into my multiscale guitar after LOTS of trial and error but learned soooooo much along the way.