Hi belli - sorry I didn't get around to saying welcome back!
For me, the only test that stands logical scrutiny is euphony - if it sounds decidely right and normal it is, and if it sticks out it may be better avoided. This places the onus on a personal and imprecise assessment of course, which is what I suspect most prescrips are psychologically running scared of but! Some have a tin ear and some have a natural flair - I expect your interest in both language and music will provide every bit as useful a guide as any style book.
fwiw, your example may confuse since you appear to be suggesting two variants one of which is just a comparative ('more') and one a superlative ('~est'); that apart, my ear would suggest avoiding securest, perhaps since lacking a stress on the first syllable seems to make it unbalanced.
