Many coaches on the Forum do not like to prescribe times … for a reason. We don’t know in what form your athletes are NOW - will they be able to run a 50" 400m NOW?? Do you want them at sub 50 at the end of the season? When is that?
But, I will try to assist!
For the 6x200 session, my recommendation will be the following: This is a VERY hard session for athletes. I normally start with 5x200m - and sometimes, at the beginning of the season, even with 3x200/2x200 with a longer rest in between. Your idea of 26" is the same as what Kitkat wants from his athletes - the same time as the SECOND 200m of the race. (You will have to adapt if your goal is sub 50.) If you think your athletes will be able to complete this session at 26", and you have to switch to 150m (for the reasons given), your idea of 19" will be fine. Start off with 6x150m and when they can easily do it, either make the intensity higher OR do 8x150m at 19". (To have progression.)
The most important thing to remember, is - YOU, as the coach, will have to be in control … ALWAYS … of the programme (BELIEVE in what you do) AND in the decision of what you expect from your athletes. These are only guidelines given!
I am sure you will make a BIG success of the coaching!! Your ideas of what you want to do -with Kitkat’s help! - and at which intensity, were all good.
Take care!