How good/bad an idea is it to substitute Manila clams for littlenecks in a spaghetti-with-clams recipe?

In fact I vastly prefer Manila clams or cockles to little necks in these types of preparations. My personal preference is always to go with the smallest clam, which to my taste at least are much sweeter. Maine goldens are also great.

I've been using a tin of clams which also gives me the "nectar", and add a handful of clams in the shell for garnish.

I've done this many times and Manila clams are a great substitution. Nothing else in your recipe needs to change

Should be totally fine!