When setting the amount of an ingredient (RMB on an item on the right side of the table) then FP will always jump back to "Items / Minute|Second", which is a bit bothersome when you wanted to with the amount in Belts.

Not only do you have to change the unit back afterwards, but you also can't even enter the amount of belts.
By now I know that 1 Yellow equals 900 / min, but still, for bigger amounts than my tiny setups I think it'd be nicer if you could use the same dialog here like when adding a product, where you can choose x of Belt.

And speaking of the product adding: could a field be added that works like "x of Belt" but instead it's "x of Assembler"?
Basically you choose an assembler type and then the amount of how many you want to have run at 100% for this?

Having that dialog work with belts is on my TODO list, so I'm definitely planning on getting to it at some point.

Regarding your second point, I considered it, but since you don't necessarily know which recipe you'll use when adding a product, and thus not even know the category of machine, I decided against having that option when adding a product. It's just too messy. You can however set a fixed amount of machines after adding a product and its recipe by right-clicking it, setting a limit and enforcing it. That will make it so that recipe produces exactly enough to use up the number of machines you indicate.