One thing all newbies go through it "coupler angst"....
So you need to figure out if you want to standardize so everything couples to each other, or live with the fact that only certain locos will couple with others.
Once you have figured that out, if you decide to standardize, then you will also need to standardize on coupler height.
This can be driven by economics, the scale of the loco, how close to prototype you want, etc.
An example: I run mostly 1:29 and model mainline trains. So I needed to standardize and I went Kadee. Now I wanted remote uncoupling, so you HAVE to use their specified height because of the magnetic trip pin.
But I know people who standardized on USA Trains knuckle couplers, and USAT has a much lower standard coupler height. This person was mostly USAT, so he only had to modify a few cars that were USAT.
I know another person who loves all steamers, and buys different brands, and the coupler heights were all over the place (LGB, Bachmann, etc)... He decided on a prototype height based on 1:20.3 and did all the couplers that way.
So, you might want to think about your long term plan, and modify locos as you go.
Greg