First off, I wouldn't recommend using the script templates from PKUSV. Use XSE instead.
Second off, I would write a script like this in XSE, and use it through a script tile
Code:
#dynamic 0x800000
#org @start
lock
msgbox @t1 0x6
compare 0x4011 0x1
if 0x1 call @battlex
compare 0x4011 0x2
if 0x1 call @battley
compare 4011 0x3
if 0x1 call @battlez
msgbox @t2 0x6
setvar 0x4012 0x1
release
end
#org #battlex
trainerbattle 0x3 0x1 0x0 @win
return
#org @battley
trainerbattle 0x3 0x2 0x0 @win
return
#org @battlez
trainerbattle 0x3 0x3 0x0 @win
return
#org @t1
= yourtext
#org @t2
= yourtext
What happens here is you set the variable 0x4011 to a value when you pick your starter, so you can set up a battle, rival or otherwise that depends on that var. The var chooses the battle based on the starter, for example 1= grass chosen, 2= fire chosen 3= water chosen. It's difficult to explain, but it's the way I'd do it.
Third off, check out this tutorial series:
Link it's how I learned how to ROM hack.