Here's something to blow your mind:

When Ifrit was introduced people only wanted Lancers for it because it can attack while being just out of seer range. Fast forward, people decided LNC/DRG was no longer viable for the fight for some odd reason and only want THM/BLM setups.

Now, in Ifrit Extreme, we're always using DRG again, yet people still shout for BLM only for Hard (and Extreme people are trying to do it as a BLM setup even though pretty much every victory I've done was a Melee setup.)

So no you can't "make your own party" because the XIV community is so coddled by easy mode they refuse to deter from it, which is the one of the reasons 2.0 will fail and why SE adding new classes and jobs is a waste of time. You can make your own party if you wish, but you can be guaranteed you'll get less people or people to stick around if you're doing anything other than WAR/WHM(2)/BLM/BRD setups of varying numbers.

People also say "well do it with your linkshell."

Unless it's a linkshell you created or were in that isn't comprised of the general XIV community, you even find "THM/BLM ONLY!" in shells as well because "it's most efficient". Though to be fair, 1% of the blame falls onto SE making speed runs which people find BLM stacking best for.