So I relocated the Fuse Box to where the coolant reservoir was and had to extend 2 connectors (the connectors that plug into the transmission) and used 18Gauge wire. I’ve noticed the wire on the harness seems to be 20Gauge. I’m just wondering if that’s okay since there’s more strands on the 18GA (my weakness is wiring, although I did use shrink tubes and soldering wire ). I’ve only used about 10in as well.

I know where I relocated the fuse box is not the best place, so before I get flamed, I did fabricate a cover to cover the fuse box so nothing can get in .

Simple answer , yes , a short 10" splice of a 18 # connected to a ckt's 20 # is fine , and soldering that size wire is superior to other methods (electricall / mechaniclly). BUT , with all due respect from your mention " my weakness is wiring " , I will say here 18 # is BIGGER than 20# , you knew that ? great .

But IF you've got it backwards and used a next smaller wire size (skinnier) , some may think this always a no-no . Not so due to short , 10" , AND something in wiring called " tap rule" theory - which is more officialy used in the NEC (Natl Electric Code) in buildings etc.Also why light fixtures,appliances got skinny wires inside too,etc.
Gotta really be versed in this stuff to consciously USE these options properly and safely though.