Sweetpea - your welcome.
I can try and answer about those painters, but I have to check with the painting experts I know.
But Michelangelo is not thought of as someone having a very good painting technique. Many people at my school consider him to be a crap painter, and it's true that he really hated to paint - he only did the sixtine chapel by order of the pope.
But as a figure draftsman he is one of the most advanced, perhaps THE most advanced ever to have lived, except he didn't draw girls or women.
He invented many crazy poses, some almost impossible for a model to take. This requires a huge anatomical knowledge and mastery of constructing the figure from imagination, and then just endless work(it's said he worked 16-20 hours a day).
But all the artists you mention were superb figure draftsmen, I honestly don't believe many 19th century artists could compete with them when it came to drawing the figure, but that's my own opinion. Of course the 19th century was much focused on realism, value study takes time away from other aspects of technique, so does study of realistic flesh colors.
As for the painting, I don't know that much.
I know that Leonardo used glazing techniques, so did Raphael. And their technique require very good rendering, doing bargues and plastercasts wouldn't be bad.
And Raphael used many assistants as did Michelangelo when he was painting the sixtine chapel.
I believe all the old masters used hired assistants for the hard but more mindless work, something we don't hear so much about.
If anybody else can help answering those questions about their technique I'd appreciate it.
But if those are the artists you love above all I would definitely suggest a strong focus on construction, studying with Glenn Vilppu wouldn't be a bad thing.
When it comes to artists using assistants this sculpture has a story to it. It was done by Bernini in 17th century Rome, but he didn't do the detailed leaf-work, that was all done by his assistant(Giuliano Finelli) who had specialized in detailed marble carving.
But Bernini did take all the credit for the sculpture and that credit has remained his to this day. Needless to say that his "assistant" got a bit upset, it might actually have started as a collaborative project.