I think you're getting slightly confused about Freebase vs Nic Salt and VG/PG ratios.
Shortfill juices tend to be at 70/30 or 80/20 and are designed to have 1 18mg 10ml nic shot added to a 60ml bottle containing 50ml of juice (or 2 nic shots to a 120ml bottle containing 100mls) so you get 3mg juice which is the most commonly vaped DL strength. You can add Freebase OR Nic Salt type nicotine shots to them, depending on your preference.
Most premade 10ml MTL juice bottles will be 50/50. Although many are nic salt at 10mg or 20mg strength there are plenty of premade Freebase MTL juices (most commonly in 12 or 18mg strength) if you want to stick with freebase nic for MTL.
Basically you can vape DL juices with Nic Salt type nicotine or MTL juices using Freebase juice if that is your preference.
What is very important is that you get your head around VG and PG ratios. Coils designed for Direct Lung Vaping tend to be much larger than MTL coils and have correspondingly larger juice inlet ports so they wick freely. If you use 50/50 with these coils they can (and probably will) leak juice and dump it over your mod. 70/30 juices are thicker because of the higher VG ratio and are more suitable for DL tanks as it will be less prone to leaking (80/20 is also available, but 70/30 is the most common type in the UK)
On the other hand MTL coils tend to be very small and have much smaller juice inlet ports than DL coils. Generally 50/50 will work best with these coils because the higher PG ratio makes the juice thinner so it will wick freely through the smaller inlet ports in MTL coils. If you use thicker 70/30 with small MTL coils you run a risk of dry hitting because the thick juice can't wick fast enough or burning out the cotton if it gets too dry.
It isn't an absolute rule - some MTL coils can wick 70/30 juice (but don't try 80/20 as that is pushing your luck way too far) but most will struggle with thick juice and your coils probably won't last as long. I wouldn't risk using 50/50 in DL tanks as juice leaks can get everywhere and if thin juice gets inside the mod it can fry the board and you will end up with a paperweight,
If you want to try MTL vaping I recommend you stick with 50/50, but buy a few 10ml pre-made juices in Nic Salt and Freebase types to see which you prefer. Just buy one of each - you will quickly find out which type you prefer.