Fin placement for a twin is not the same as a thruster fin placement, and single fin with standard thruster fins will be very slidey. a true single fin usually have the standard 10.5 " box or smaller boards have 6.5"-7.5" box's
Edit: if you want to start doing a little board building early, from your other thread why not add a twin box and remove the center box and add a longbaord box and get the FCs adapter to fit thruster fins in the longboard box. could be a nice little project before you build your first board.
I have a thruster I want to take the two front fins off to make it a single. Or take the middle fin off to turn it into a 2 fin
The nice thing about having removable fins is you can do whatever you want!
Take all three fins to the beach and try both! My guess is you wont like it as a single fin, in small waves you might like it as a twin fin (even though the guy above is right about fin placement, but we've all done this with thrusters), but most importantly, remember to tighten the set screws each time you switch so you dont lose a fin or a screw.
You'd have to do like A2tall says and add a box....A thruster trailer sits back farther than a single fin does on a board. Not sure how that would ride the way you are talking about.....most likely pretty squirrely, but then you might like it. Are your fins glassed in by chance???? Cause you talk about riding a twin and if it has boxes you would just remove the trailer......
Twin fins should be about 6-8" up from the tail this varies depending on board shape, and what you want to do with the board ie some simmons have their twins 1" up from the tail. The purpose of the single fin box is because when only riding a single fin you want more surface area on the fin to hold you through the wave, notice how their is much more surface on a most single fins, and when you get twin fins they are usually larger than standard thruster fins, and quads have smaller rear fins. it is all based on the surface area and shape of the fin which balances hold, drive, pivot...ect. if i was you i would add 2 box's @ about 7" up from the tail and that way you can ride it as a twin with the rear plugs if you set them right, a quad keeping your standard thruster setup for front the the rear ones will be the twin/quar rear fins, remove your center box now and add a 10.5 box get the fcs adapter so you can ride thruster, single fin, 2+1 quad, twin ect... it is called the versa plane setup.