Ya it's hard to figure out how much is too much salt or added fats. It seems easier most of the time to get either single ingredient foods, or very limited ingredient foods. Making simple smoothies, hot cereals that you can add your own goodies too, ect.
Foo was my food baddy when I got her. She was on a seed diet and it seemed all she did was fling everything and eat the sunflowers.

It took about 3 years to completely get her on pellets. Discovering nutriberries was like the best thing. She took to those right away. From there I got her on roudybush and she is finally eating some harrisons as well.
Veggies are still really hit and miss. She won't eat chop, she only likes veggies and fruits she can pick up and they need to be fresh and crisp, and she is very picky about what ones are good and she is prone to change her mind. She does like sprouts and peppers a lot though so that helps. She loves almost all fruit. I'll take finally done chop or just blend it and I can stick things she loves in it so at least some of the mush that clings to her tastys.
Ellie is much easier and I just got her even though she was also on a seed diet. She is at least willing to try everything. She's already eating the pellets and she loves mushed up veggies and fruit, so I can sneak all kinds of things into her diet. If I hold the bowl she will eat anything with enthusiasm. Though she is opposite of Foo and doesn't want bigger hunks.
Everyone loves crackers and pasta though. Gotta love those carbs.