Tetraploids and hybrids are cr#p for grazing gone in year 2 or getting very thin.There is no problem at all with having Diploids in the mix and sowing it now. Ground conditions are ideal with very good soil temperatures and as we get closer to August the possibility of more rain.
There are more than double the seeds in a kilo of Diploid Perennial Ryegrass (600,000) than there is in a kilo of Tetraploid hopefully some will grow.
The Diploids are there because they have more tillers per plant meaning they will have better ground cover than Tetraploids. Due to the lower water content per cell, diploids have a higher dry matter per kg of feed, meaning they cure and wilt easier when making hay or silage. They also offer more energy than tetraploid plants.
This is a permanent mixture Hybrids aren't going to "cut the mustard".