I found this comprehensive list will come handy.
See URL

What kind of midimessages can i expect to be common between "most early GM moduls/synths"

Everything in the GM Spec is what you can expect to be common in GM devices.

Which of those messages is part of header of midifile, and which can i find continous within?
I guess but are not certain can a program change be part of a midifile, a bankchange?

Are they part of header of midifile or do not belong within it?

And i am not sure that all the messages are in common, i meant more which "functions will be readily available", for example my roland SC-7 do not have banks so no bankchanges, so it does not have that in common with more advanced devices. i doubt it have aftertouch?

It do have reverb, chorus, pitchbend, modulation and mastervolume reset e t c.

Your questions about the MIDI file header tell me that you have not read the MIDI specification for MIDI files.
The spec is freely available. Click the "Specs" link at the top of this page. Get the full MIDI specification, it includes the MIDI file information you need to be able to correctly read and write MIDI files.

There are no bank changes in GM. As long as your SC-7 implements the minimum specs for GM, then it is a GM device.
Bank changes were introduced in GM2. If you have a GM2 device, then it will support bank changes.

Every message type can be sent at any time. You choose to put a setting at the beginning if you do not change it during the song.

The GM specification tells you what the minimum requirements are. There was hardware that was designed to just barely meet these requirements (e.g., OPL4); for a current example of a minimum sound module, see the VS1053b datasheet.

Hello,

A lot of this thread is really VERY LITTLE to do with midi files as such, and really to do with the facilities available to the receiving device(s) and with the options that the sending sequencer might send.

Note that any device that receives a message that it does not support should disregard it, so there should be no problem. Other than that the sound created may therefore not sound like it was intended to!

My setup includes a Roland LAPC-I card, which is pretty antique now, and is certainly pre GM. However, this does support pan as a CC (Continuous Controller) and I have midi files that include pan, and some pitch bend, that do affect the music. But there are no banks, no sustain (although some things could be controlled via SYSEX acting on a specific patch during playback). I have midi files that sound great played back using the Roland card, somewhat less so if played back using a much more modern device.

I also use a Yamaha MU90r, which is much more sophisticated than the Roland card, including full GM and XG support, banks, reverb and various other things available as CC. I have midi files specifically targetted at such devices, which do not sound right at all if played back on anything else.

If your setup uses a specific tone module (i.e. the SC you mention) and you are creating your own sequencer, then your best option is to ensure that the sequencer fully supports all the options available to your sound device, this will maximise the benefit to writing your own sequencer. If you then need to play midi files created for a different set-up, you may need to make some changes?

Geoff

Well i sm slowly moving forward to what George say a full support, i guess in a way there already is because it echoes and record anything that goes in.
But of course i try to implement a graphichal interface, a control interface? and i wanted to know what kind of messages such an interface should deal with, but maybe it so simple that no interface fit all ends, and that every device simply have to have its own interface file.

Right now i have just implemented chanel vol, pan ,sustain, modulation, reverb and chorus as generic features. But i can see in spec of SC-7 i can set type of reverb.
I just wanted to know if thee were any more generic features i should add "to setup play/record" using maybe a slightly older keyboard as controller "that lacks the knobs and sliders" of modern controllers.

https://midisequenser.000webhostapp.com/

I also was thinking let my input deive UMX490 controller keyboard "buttons, knobs and slider be remappable to the output device functionality "CC message" via a gui where one chose the CC message for knob,slider,button via dropdown list and then it is remapped before sent out to device, the way you want it linked to fit your liking and functionality of output device..

For example i hate the UMX do not have default program change and bank change using button up, button down..
So that is how i am thinking remap probably the transpose buttons" to have choice.using radio buttons "program, bank, transpose"

but maybe it so simple that no interface fit all ends, and that every device simply have to have its own interface file.

I think you have covered the basics. As you have discovered, additional features would be device specific.