So if you built an MG turret you could change it to ML turret for 50 res, and if it's an ML turret then back to MG for 50 res. And just so it doesn't have too much advantage, make it so it takes 30 seconds for each turret to change itself. EDIT: Also, to make it even more fair and more dangerous, make turrets inoperable during the 30 second interchange time
Make it less - 25 res. It takes 75 to place a ML and 50 to place a MG I think. If you have a change time penalty, there's no reason not to allow it for less cost. Or at least cost of MG=>ML.
No, the purpose of turret is preparation. If you want protection against both infantry and vehicles, build both types. If they destroy all your MGs, too bad.
Implement bacon dispensers, make them turn the friction in the given area off, as well as slow the movement speed of troops within it's area of effect. Make troops die if stamina reachs 0.