If they were to limit what bomber could lift from what field, wouldn't bother me any. I thought it odd that a heavy bomber could lift from a small field. Lifting from a medium field I would think is pushing it as well. While it may or may not be good for game play, I personally wouldn't mind if they did the following:
Small Field: Light Bombers only.
Medium Field: Light and Medium Bombers only.
Large Field: Light, Medium and Heavy Bombers.
But what about the following?
Small Field: Light, Medium Bombers.
Medium and Large Field: Current.
As for the grass runways, what if the ability to knock them out was added as well? So you'd have your standard fields, but the small grass strips could be knocked out for 30 minutes? Reason I lean a little heavy is because you'd still have the main field nearby to contend with. Mind you this is based off the idea that they are tied to a nearby base. As for what can support what, I'd think the Large fields could easily support one or two. The Medium fields could probably support one, and the small fields are on their own. Ports could probably support one but GV bases could not, though adding a little grass strip for the Fi would be a nice little touch. But that's just eye candy.

As for what could be loaded at these grass strips, I'd think no ords, but the ability to take 100% fuel should be in. DT's however are in the air. I'd say no to DT's at the grass strips though.