As long as they are not overpowered compared to the stock parts. For example, if you had a fuel tank mod. The fuel to weight ratio should be the same, or under the current parts. As for plug-in based parts like KAS, If you think that they are fun or "Should be a default thing" then use them. Mods that I recommend are B9 aerospace pack(The one you mentioned obviously) , Kerbal attachment system, Kerbal Crew Manifest, and Kethane(The mod hasn't been officially updated for 0.20, but you can find some forum users that have put together a decent patch for now.).