I'm not sure how to answer your question, because I don't understand it 100% but I might just answer it in two ways and maybe you'll find what you need in there. I think Answer 1 will be right probably...
1. Writing a "suite" or just lots of music before starting a project is a great way to find a general tone for the film. I think Hans Zimmer does it most of the time, as a way to find the sound of the film and maybe also as a Proof of Concept or a form of Research and Development. At the same time, you're also "practicing" how to write for the film, so I see nothing bad in that.
The only thing is that you have to be careful that not everything sounds the same when you're writing the actual music for the film. Try to develop different sounds, moods and themes during the research process. Just try things out - this is the time to go into different directions and see what fits best. And don't just randomly choose things from that material afterwards to make the score, try to still adapt the music to the scenes to make them fit right.
(Yes, lots of obvious and no-shit-Sherlock stuff here)
I've done both writing with and without a suite. I like both ways. I think there's no "right" way to work in this mode, so just go ahead and try to write something. Maybe if you have the script, try to imagine some scenes and write some music for those. Or write some pieces in different moods that the film conveys. I've heard of many people that work in that way and that save a lot of time because they just have to adapt themes and pieces to the existing picture when it comes in. But I think its main advantage is finding the tone and the sound of it and maybe writing themes and the actual music that goes into the finished project in the actual scoring process.
Or do you want to write a suite of different themes you explored throughout the score? Maybe you want to have a suite afterwards that goes onto the score as well. In that case here are my thoughts about how to write a suite afterwards or just writing a suite in general:
2. I always like to think about it in a classical symphonic sense, especially thinking about the sonata form. I try to have a beginning, a middle part and an end in everything - and more importantly how the overall arc feels.
When I start, I listen to all the pieces and look at which are "interesting" and maybe even melodic enough. You don't want to bore the listener with a 30 minute piece of the same ostinato. I choose the most interesting parts, the most memorable parts, the parts that represent the score the best. Then I roughly arrange the order in which the pieces appear. In my experience, it doesn't help if you have grandioso, quiet, big, small, action, quiet again after one another - it will sound all over the place. And also, hearing two versions of the same piece doesn't work in most cases in my opinion. Remember: Music doesn't have the same effect on the listener when listened to without a picture opposed to hearing it in a film context. Keep it interesting.
Maybe start with an intro, present the main theme or one of the main themes, have a B section (maybe quieter, smaller, etc.) and finish big - maybe a reprise or another big section.
Another way to approach this is to have a constant sense of building, start quiet and add more and more momentum throughout the different pieces you're sticking together. But remember, keep it interesting.
Good examples for suites are mostly the end credits for movies (I especially like the end credits for the three Fantastic Beasts films as James Newton Howard is also a master of composition in a classical sense and knows classical form pretty well). In my opinion, almost every John Williams Main Title or Main Theme is a wonderful example for a suite. Almost every Main Title is essentially a little suite, presenting the themes that will appear in the movies. Think Raiders March - you've got the Main Indiana Jones theme, then the Love Theme and then the Main Theme again. Masterfully and perfectly executed. Or his end credit suites for the Star Wars films. And also the "proper" suites by J. Williams just feel right. The arc is right, the overall drama is right, there's a story to it.
Just like with every piece of music you write - tell a story!
