The other feed (if it's Purina) sound like it's Flock Raiser which is 20% protein. It's not a layer feed as it doesn't have the added calcium that a layer feed has.
The reason you would not give a lot of treats when you're feeding Layena is because for chickens to lay well they need a diet containing 15-16% complete protein. Scratch feed typically doesn't have a very high protein content. BOSS is somewhat better, but that protein is incomplete. If you offer a lot of low-protein supplements it dilutes the total protein content of their diet which in turn will cause egg production to sag. If the birds are free ranging this is somewhat less important as any decent kind of range generally provides a fair amount of protein but not typically a lot of carbohydrates. In that case some extra treats won't do any harm. If the birds are in any way confined though then don't feed a lot of treats if their main feed is Layena.
Flock Raiser or any other kind of ration that is 20% or more in protein was designed to be supplemented (so far as chickens go) and so long as the supplements are palatable the birds will do a pretty good job of balancing their diet. I use a lot of Flock Raiser and what I do is hang a feeder full of it and a feeder full of mixed grain that the birds can choose from free-choice. Typically they'll eat 50-60% grain and 40-50% Flock Raiser which puts their protein intake at roughly 16% which is what they need. I like Flock Raiser because it cuts down on the number of different kinds of feeds I have to keep on hand. It doesn't have additional calcium though so you'd need to offer free-choice ground oyster shell or something like it on the side. I usually hang a small hopper of it where the birds can get at it and they eat it when they feel the need.
